No. Once the fork occurs, they will forever be independent of one another. Transfers and custodial grants will only effect one network.
Yes. One issue with this is that we wouldn’t be able to give any blockchain assets at the conclusion of the auction until B&C is complete. It is also possible to fork the Nu chain now to create a B&C chain, but then we would have to do all our development on a live production chain. That would require more resources and place the production system at unnecessary risk.
Using NBT as a dividend currency is practical from a technical standpoint and I agree there are advantages to using a stable currency for dividends. However, the success of cryptoassets has much to do with the number of people using the network. The selection of Bitcoins as the dividend medium is the optimal choice for encouraging broad network participation.
How many confirmations an order must have is a network wide variable configurable through shareholder voting. 3 confirmations (typically less than three minutes) will be a likely shareholder choice.
Trades do depend on reputed signers signing multisig transfers to settle. This will take seconds. Reputed signers are not necessarily shareholders, although they will likely place a security deposit of BlockShares.