1.) I don't think that will work. As far as I know you must have a FUNDED account on stellar.org net to setup trades on the internal stellar.org network trading system
The network is also not presently setup to do automated bank transactions direct from fiat banks at this time. At this time that still takes some transaction time like up to 24 hours.
On the other hand a prefunded trade that is already setup on one side could in some way be instantly hit when the bank transaction is finally performed on your side B.
so maybe half your idea will work?
2.) Transaction fees to move assets from fiat bank to stellar.org and from stellar.org to fiat bank is determined by the anchor. each will be different some maybe free. so look at the docs of the anchor to determine this like for funtracker.site you will see fee's for each operation stated here: https://www.funtracker.site/wiki/doku.php#funtrackersite_bank_fee_s_for_services that you can note deposits are free, withdraw from stellar.org net to fiat thailand banks is 0.05%. details: Fee for a Withdraws: through the Funtracker.site Bank gateway with direct transfer to any bank in Thailand (THB only) or paypal.com (THB or USD) or BTC bitcoin (BTC only) transfer: 0.5% of the transferred amount, fee is not to exceed 200 THB (~$6USD) value max per transaction. In most cases you will be asked to send the issued asset amount of withdraw to stellar accountID:
3.) As far as tax, taxes are handled on the users side at least here in USA NOT by brokers or banks. The Only time the BROKER (not the bank) holds back tax is when the account is registered as an IRA tax deferred account. This might be different in other countries I'm not sure. My brokers at the end of each year provide me the data needed to submit my tax report to federal and state tax submitions. Maybe some day our trading apps or other app will provide this service or feature. It would not really be hard to create an app that all you would have to provide is your publicId. from that it could search all trades made within the calender year and determine profits and losses and create data output that is compatible with tax programs that handle these things.
4.) As far as how stellar.org transactions scale. Stellar.org network has been measured at handling about 1000 transactions per second that is equal to 86,400,000 transactions per day. That number should provide you some idea of how much we can handle. I assume you can do the math your self on other windows of time with different numbers of people involved. Also in the event that is not enuff there is no reason we can't have 2 stellar.org nets or X stellar.org nets that are bridged together as almost one. In this manner there is no limits to number of transactions per second.