Q1. does this bid mean the starting price for batch 3 is at least $500? if so, wow!

Yes that's exactly what that means.

Q2. what if the bidder cannot keep this promise afterwards? (this is a question in general, not referring to svamiva in particular!)

Then I'm going to be sad and my feelings will be hurt. There isn't a mechanism here to force settlement. We just assume honorable intent.

Q3. the number of units - does it need to be integral?

At settlement you can specify number of units. VIVA Crowns are all integrals because they are a specific right within the system. So the number needs to be an integer, that integer can be as low as 1. No reason you can't get a group together and subdivide an interest, but that's a personal business dealing.

Excellent questions and thanks for bidding!