[Estelyn Telcontar]

Rather than offer my own opinion, I'd again like to draw on Tolkien's text to deduce answers to those questions of yours, alatar.

Let me start with your "making of the Seven" - as I said, I see no evidence that the Seven or Nine were made with a specific number, race, or purpose in mind, nor even that they were separate. It would seem to me that Sauron divided them arbitrarily; they may have been the sixteen most powerful - we don't know what other rings there were, but since they play no role in the further story, we can neglect them.

As to the purpose for making the rings, it is not specifically mentioned, but from the context, this is what I assume. Here are Sauron/Annatar's words which precede the making of the rings:

...wherefore should Middle-earth remain for ever desolate and dark, whereas the Elves could make it as fair as Eressëa... ...I perceive that you love this Middle-earth, as do I. Is it not then our task to labour together for its enrichment, and for the raising of all the Elven-kindreds that wander here untaught to the height of that power and knowledge which those have who are beyond the Sea?

The passage goes on to say that the Elves received his counsel gladly,

...for in that land the Noldor desired ever to increase the skill and subtlety of their works. ...they desired both to stay in Middle-earth, which indeed they loved, and yet to enjoy the bliss of those that had departed.

In other words, the Elves wanted to have their cake and eat it too! However, their motivation was positive. They wanted the power to do good in Middle-earth, perhaps even to negate some of the evil Melkor had done. So while some reasons were a bit selfish, the general idea was not necessarily so.

Whether Annatar slipped a chink into all of the rings in order to get his "virus" in there later cannot be told from the text, though it might be possible. That would not entirely account for his influence on the Three, which he did not make but which were forged with the knowledge he had imparted to the Elven smiths. However, it is quite clear that the rings themselves would not have had that effect on their bearers without his influence. It starts with the fact we already know:

...secretly Sauron made One Ring to rule all the others, and their power was bound up with it, to be subject wholly to it and to last only so long as it too should last.

Later on, when he has regained all but the Three, we read:

...all those rings that he governed he perverted, the more easily since he had a part in their making, and they were accursed, and they betrayed in the end all those that used them.

It would be interesting to speculate on what would have happened if the Rings of Power could have been used by the Elves without Sauron's curruption! Would the desire to make Middle-earth like Valinor have caused regression instead of progress, like we see in Galadriel's realm?