I answered a similar question a while back. While the cult of the saints certainly fills a niche once serviced by pre-Christian cults ,that of the supernatural patron, I would not go so far to say it was a direct channeling of Roman polytheism. At least as far as origins are concerned, the practice has more in common with late ancient funerary rituals than the rituals surrounding the cults of heroes and gods in Roman society. The emphasis on death and suffering as a vehicle for obtaining proximity to the divine was an important break from earlier traditions. In a similar vein, the Christian habit of partitioning relics, parading them around, keeping them on their person, etc. deeply disturbed 'pagan' contemporaries.

Now you are correct, the cult of the saints was wildly popular throughout the early medieval period, and at times does seem to take on a role in popular religion once occupied by gods and other supernatural beings. However, I would argue this has more to do with religious and cultural syncretism than a deliberate and coordinated attempt to make Christianity more palatable to the unsuspecting masses. Saints seem to fill a social and cultural niche as supernatural intercessors, a source of 'magic' that individuals might be able manipulate to their advantage. The manipulation of the supernatural was nothing new in the late ancient-early medieval world, but for various reasons, many gradually adopted Christ and his saints as the more viable or potent option. The extent to which theology mattered to everyday converts is difficult to understand, but conversion was usually not an either/or decision; people typically don't discard a lifetime's worth of cultural vocabulary, so it is not surprising that at times saints seem to adopt the vocabulary and tropes of their more 'pagan' predecessors.

Basically, the cult of the saints was something new, both in the way it was articulated by Christian theologians and the manner in which it challenged long established taboos regarding death and access to the divine, but that does not mean it can be divorced from the cultural and religious context it was heir to. However, the cult of the saints and those who wrote about it were not a monolithic hive-mind and the degree to which saints resemble and adopt pre-Christian traditions varies from author to author, text to text, saint to saint, and region to region. Some saints come across as very human, and others more directly embody pre-Christian antecedents (see Brigid of Kildare and the Irish goddess of the same name). Saints were not part of a conspiracy to appropriate Roman gods, that misses the nuance of the cults and ascribes to the late ancient church a level of coordination that did not exist. Rather, when saints adopt pre-Christian elements and tropes, it is evidence to the malleability and cultural syncretism that were part and parcel of late ancient and early medieval religion.

I dont quite think so. Veneration started with martyrs, whose death were honoured in venerating them. After some time, it was extended to the apostles (which where aside of John Martyrs too), in order not to leave them without this honour.

As time comes by, the apostles overtake the other martyrs in veneration and importance because of a couple of reasons (they were honoured where they founded communitites, for example). In the 4th Century, veneration was extended to select bishops, as well as Marian devotion. I haven't read someting about it being connected to the polytheistic systems in the past, as there is a important difference between veneration and adoration and this distinction was abided.

This exact problem came up in the reformation-era: For everything, there was now a saint, and even larger groups of saints, like the forteen holy helpers. Every profession was subordinated to a specific holy person. This trend continued and these holy ones were slowly not only venerated, they were worshipped, which is against christian rules (10 commandments), and even worse it was seen as "best" to address as many holy ones as possible, hence these groups. This progression towards polytheism was one of the reason why the reformation started to happen.

(I'm German, so forgive me possible grammar errors!)