Ah ok got it, yes that for the right hand will be precision that comes from practice, something you have to make your hand learn by repetition (I saw that he keeps his hand very close to the guitar, the movements aren't big except on certain chords, so I expect this is a way to achieve that control by forcing some of the movements to be smaller on the upstroke). But, since I'm not good with picks at all, if I had to approach it, I think I would try to replicate it if using a pick by using the left hand, muting the strings I didn't want played in a given chord on a beat. Its something worth practicing in any case, left hand (and right hand - 'palm muting' which he may be doing some of given how close his hand is to the strings, but sticking with left for now) muting - you can achieve it by maintaining less pressure on a given string at a fret, so it doesn't sound properly (but not so little pressure that it makes an annoying sound). You can apply this to neighbouring strings with a finger, by pressing on the string you want to ring, and sort of flattening the rest of your finger a bit in the direction of the string you want to mute, so it presses one and also covers another (sort of counter-intuitive to how you're supposed to want to use your fingers, i.e. very precisely pressing). For chords that don't use one or more bass string, you can curl your thumb around the other side of the fretboard to mute those.

With hammer-ons, don't worry about it being slow to start. Again, with 'classical' guitar, Baroque music happens to have a ridiculous (over) use of them, called 'trills', where they shove a hammer-on pull-off combo onto the end of so many parts, and its a good way to practice them, by taking any given individual note and repeatedly hammering on or off to produce that kind of 'trill'. A good riff to practice them is the intro to 'Layla', because minus the effects its quite simple, so you can slow it down to start with and build up to the correct speed.

Another useful thing about other genres (including classical) is that playing them teaches different timings, since most music of the genre you're talking about is in 4/4, and it helps to understand that timing by also using different ones, like 6/8 or waltz etc.

Here is another area where I think 'fingerstyle' helps more, because you're playing individual strings, you kind of 'hear' and 'feel' better the effects of hammer-ons or offs compared to incorporating them into strumming patterns. I mean that your brain is, I think, better able to get what you're trying to do, because the chord + strum is comparatively 'chaotic' or a finished product, whereas with individual notes, you're showing your brain the building blocks first, so to speak.

On playing the whole song through - its partly to get a sense of the broader 'flow' of the music, but its also so that your brain doesn't get backed into a corner with a specific problem that it (or it in combo with your fingers) isn't currently able to solve, so I think its best to move on from persistant roadblocks to give your brain something it can solve (an easier part) to prevent it getting too frustrated. While you're sorting the easier parts, your brain will still be working on the greater problem, but the easy parts may help it with a new perspective on the difficult bit, I hope that makes sense. So its good to switch back and forth, and also give your brain a sleep cycle to make progress on a particularly difficult roadblock. Again, switching styles (genres, and ways of playing) helps because the differences will help your brain work out the roadblock, give it new ways to approach the problem, and keep it from getting too frustrated and giving up or going too far down a 'wrong' path solution. Brains need sensory data, the more the better, to solve problems. Of course, going back to a difficult part multiple times isn't a terrible idea, because it also teaches your brain that its a crucial problem that must be solved, as long as you're not getting frustrated or fatigued with it.

One thing I remembered that somebody told me, applicable to pick users, is to try to practice a pattern when picking individual notes, of alternating each string. So you'd play the E string (bass) with a down pick stroke, and the next string (A) with an up pick stroke, and so on. I was told this is a good habit to force yourself to learn (it'll be slow at first) because it will allow for greater speed and finesse later on, if you're picking out notes in a riff or chord for example.

A final note on theory - you often get people tell you that its crucial to learn this stuff. I wouldn't disagree, and I think theory can help you 'shortcut' some stuff that might take longer to appreciate by 'doing', and also give you a better understanding of the subject as a whole, which will no doubt be useful for the 'doing' part.

However, personally (and I don't want to contradict hundreds of years of accumulated knowledge here), I wouldn't overemphasise it. I've never tried to learn music theory in a formal sense, just picked bits up here and there from general reading, but mostly its just intuited from playing. Like I learnt chord shapes, and some scales, but I found out that there isn't really a 'D Scale' but rather arrangements of notes in sequences to create patterns that is dependent on style and cultural tradition - that is, most music theory is basically aesthetic, not ontological. Like you don't need to learn theory to paint, but there are certainly colour arrangements that people have proposed as 'canon' or recommendations, to create an aesthetic they find pleasing (like with fashion). But of course, it isn't really a hard and fast thing, and in fact different cultures have their own aesthetic rules - like a minor scale or chord in Western European tradition is considered 'sad' or 'meloncholy' but in other places (some of Eastern Europe and Balkans) it can be considered the opposite - happy and uplifting, or celebratory. People argue over the correct tuning (in hertz) of the A string.

It brings to mind for me when Ho Chi Min apparantly remarked about peasants in his country having a far better grasp of communist ideas than French intellectuals (I think this is correct, I may be misremembering) - because those peasants were doing, while the French intellectuals (aside from usually being fools) were just theorizing. Or a recent case, where a tiler discovered a particular pattern of tiles that eluded theoretical mathemeticians and was considered too difficult to solve, and of course the articles written about the breakthrough had a sort of "oh a talking dog" tone about the tiler, but really the tiler by doing understood geometry better than a theoretician.

So I don't want to dismiss it, and there is value in it, but I wouldn't worry too much about learning music theory (again, often what is meant is Western European theory - for example Jazz theory was a reaction against that classical teaching) if you don't enjoy it - you can always come back to it, and it isn't necessary to learn to play and get good at playing. The best way to learn imo is with a guitar, not with books about guitars.

I did intitally try in this reply to write you an exercise for hammer ons & offs plus descending bass line, but I think the formatting was causing issues so it didn't post with that, so sorry about that.

Edit: I forgot, but another tip that really helped me learning, is that where guitars have dots to denote particular frets (usually 5th, 7th, 12th and sometimes 9th), I found these reference points too sparse in a visual sense, so I bought small circular stickers (the kind you get in arts & crafts stores, or for children, in packs with different shapes) and stuck them on each fret, so I could better visually see the difference between frets - I don't know if this would help you, but it did for me, being able to have a bright coloured marker showing each fret, otherwise they all kind of blend and you can lose your place. I mean, its easier to quickly count four bright stickers than the alternative. If you care about your guitar's aesthetics you might not want to do this (though you can gently wash them off with soapy water), but I am function over form mostly. If I was trying to learn piano, I'd write in marker the notes on the keys, for example.