Most manual snipers use two windows rather than the forward and back buttons, and have the submit screen ready for 10 minutes or more. You're idea is logical, but don't believe it would work that way. eBay must go through all the checks again once the submit button is hit. Otherwise, we might see it quite often on sniped auctions.
I *have* seen 'invalid' bids recorded at eBay in the bid history. It does happen, however not for the reason you suspect.
Lets suppose there is an auction with an opening price of $10... no bids yet, but two snipers want to bid. Sniper A bids 10.01 and sniper B bids 10.02
If sniper A bids first, B's bid should be rejected as too low. It's rare, but sometimes B's bid is accepted!!! Here's why.
The two bids arrive at eBay very close together, (guessing here... within 1 or 2 milliseconds of each other). A's bid arrives and processing at eBay begins. If B's bid arrives before eBay finishes processing and recording A's bid, eBay will accept B's bid and 'work it out' at auction end. Both bids are recorded in the history, and B wins by one cent. If the bids are the same amount, A wins the tie, even if only by a few milliseconds.