I've been thinking a lot about this because I think you made some really good points, especially with your second paragraph. With the first I would just say that modes of production can coexist even if one is always dominant, and when one dominant mode is replaced by another, the current dominant mode can subsume the ideology of previous modes for its own ends. Italy itself was probably always the most Roman place in the entire feudal world, especially with regard to its continuous trade links with the Arabs and Byzantines, who themselves practiced slight variations on the slave mode of production. (Slavery still existed in Italy in the Middle Ages and Renaissance; some of the most successful city-states like Venice and Genoa were powered by the slave trade.) I want to say it was Cardinal Bessarion or some other Byzantine intellectuals fleeing to Italy from the Byzantine world in 1453 who helped kick off the Renaissance devotion to ancient Greece and Rome, although Italian intellectuals like Dante and Petrarch who were active a century earlier obviously helped a lot. Renaissance humanism to me seems like a reawakening and further development of the ideological superstructure of Roman slavery. The fact that you can draw a straight line from Erasmus to Locke doesn't necessarily mean that Erasmus was living under a capitalist mode of production. Capitalism itself still utilizes feudal ideas (Leonard Leo, architect of the modern SCOTUS, is literally a hospitaller); feudalism likewise used Roman ideas even in the middle of the Middle Ages (Charlemagne called himself Emperor of the Romans for instance but there are many other examples).