bracket orders (having a parent order) are activated only when parent order gets filled and only in the filled quantity (they are also updated when the parent order is filled partially and then filled completely), so 3) should not really be an issue.
for safety, i'd use gtc orders in the bracket rather than day orders, as in case something goes wrong, the orders will be eventually filled the next day in case of gtc, in case of day they just expire and you might end up with an open position, thinking that your positions are closed.
you should put your bracket orders in an oca group, so that only the total filled quantity of the parent order will be closed (in theory, you might get fills on both orders and get an overfill). it's probably better to use the blocking variant here as it would handle the case when both your bracket orders become marketable at the same time.
