Hi Anthony,
You make some excellent points. Let me try to build on some of them.
1. We (the first time reader, and the Counsellors) have more evidence about the effect of the Ring on people than the two you mention. Bilbo had the Ring for quite a while and had worn it near continuously in the Halls of the Elven King, yet it was Bilbo who tried to stop a war with the help of the Arkenstone. The Ring does not seem to have corrupted Bilbo. No evidence that we have seen that Isildur was heading towards becoming a new Dark Lord when he held the Ring. Gollum was certainly evil. It is not clear how much of his Ring acquisition was recounted to the Council (none that we can see, but perhaps it was covered by Frodo? It is, however, certainly known to the first-time reader.) However, it is not very clear whether the Ring corrupted Smeagol, or whether he was corrupted already. Gandalf's refusal to take the Ring (assuming it was reported verbatim to the Council by Frodo) seems a very personal concern. Is there any reason to universalize it? Bombadil seems unaffected by the Ring, and Gandalf says that it has no power over him. Finally, Frodo, though he reports some strange interactions with the Ring, shows no signs of incipient corruption or Dark Lord inclinations.
From this evidence, could the first-time reader, or the Counsellors suspect the danger of the Ring corrupting anyone who bears it? It does not seem likely. Note that when Erestor suggests turning to Bombadil for help it is not because he thinks Tom immune to the corruption of the Ring, but that he thinks that Tom has power over the Ring.
2. The first-time reader knows very little about the Valar. Indeed, even the word 'Valar' is unknown. How much each of the Counsellors knows of the Valar is also unknown. Is there any reason to suspect that the Ring might corrupt the Valar? Well, there is no really good reason to believe that the Ring corrupts at this point. Plus, we don't know enough about the Valar to know if they are corruptible. From the little we do know (Elvish songs to Elbereth. References to the 'Elder King' in 'Earendil was a Mariner') there is no reason to suspect this. For all we know, the Valar can destroy the Ring, or hold it forever safe from Sauron. So, the thought that the Ring might corrupt the Valar is unlikely to occur to the first-time reader, nor to most of the Counsellors (though what Elrond, Gandalf, Glorfindel might have thought is unclear).
3. If the first-time reader, or the Counsellors are worried about a bearer of.the Ring being corrupted on the way to Valinor (and no reason to suppose that they would be worried about this), then why not suggest sending the Ring over the Sea carried by Tom? Of course, I'm not at all sure that Tom would agree to this. At least not without a guaranteed return ticket. "No way! Ho Tom Bombadil is a merry fellow. He won't get put in a boat to sail the starry billows!"
So, I am not sure that the first-time reader, or the Counsellors, have any awareness yet of the corruption dangers of the Ring. Neither am I certain about what they know or don't know of the Valar (well, I'm pretty sure of what the first-time reader knows - which is almost nothing, but not sure of what the Counsellors know).
Now, Elrond presumably does know about, or believe in, the corrupting power of the Ring. He is shortly going to make a lengthy comment on that subject (though he hasn't yet). How he has come to that knowledge or belief is unclear. One hypothesis is that he gathered it from some comment of Saruman's? (Saruman being the one who has studied the Ring, and Saruman being curiously referenced when Elrond does comment on the corrupting powers of the Ring - though it is not at all clear that the Ring corrupted Saruman. )
So, why does Elrond assert that 'Those who dwell over the Sea will not receive the Ring'? I don't think that the Counsellors or the first-time readers have evidence to answer this question. Now, those who have read the whole LOTR, and perhaps all the rest of the Legendarium, might have more evidence? But still, I think we will need to fall back on supposition and conjecture, probably of an Ardaesque theological nature, to come up with satisfactory possibilities?