Interesting premise nokhor.

A couple of counter-points:

1) Gandalf did cross the River Anduin from Minas Tirith to the Black Gate to fight in the final battle there.

2) The Ring Wraiths were crossing the river when they were unhorsed by the flood, regardless of whether it was natural or created by Elrond.

3) Helms Deep was a place of refuge for Rohan in the past, so going there wasn't out of the question. Besides, Sauruman defeated a large body of Rohirrim twice out in the open at the Fords of Isen - the first of which resulted in the death of Theoden's son. So, fighting Sauruman in the open wasn't going to work.

4) Regarding the ploy to get all his enemies on one place to defeat them in one battle, that doesn't work either. In the time around the whole Minas Tirith business, Sauron was fighting elsewhere too. He made three attacks on Lorien and was repelled all three times by Galadriel and the elves. There was also a major battle fought north in Dale. Sauron won this battle, killing King Brand and King Dain Ironfoot and besieging the surviving men and dwaves in Erebor. Sauron's forces were also defeated in Mirkwood by Thranduil (Legolas' father). In fact the second assault on Lorien and the battle in Mirkwood took place on the same day as the battle of Pelennor. See Appendix B, "The Tale of Years" in The Return of the King.

5) Boromir would have become a slave to the Ring had Gandalf chosen him to carry it. Just look what it did to him being on close proximity to it. Hobbits were the only people that seemed to show some manner of resistence to its seductive powers.

6) Gandalf had the elven ring Narya which would have been quite a feather in his cap just returning that to Sauron.

7) This is more speculation than the others above, but regarding the eagles, the Ring was forged in the Cracks of Doom which was inside Mt. Doom. Simply dropping the ring in crater at the top probably wouldn't have sufficed. Also, the eagles flying would have drawn the attention of Sauron and apart from the Nazgul, we know little of Sauron's air defenses, physically or magically. He had no known dragons or balrogs (assuming they could fly).

That's enough for now. Thanks for the though exercise nokhor.