I'm in a different situation (using a spreadbetting account) but I've been through the subject as I typically trade forex with a large number of open positions and pending entry orders. Some of my firm's restrictions might be applicable.
Firstly, if one of my pending order's entry prices is reached by price movement but execution would exceed my margin, the order is never executed and is cancelled. I have seen the platform also cancel other pending entry orders (at unreached prices) simultaneously but the platform doesn't seem to apply this policy uniformly.
Secondly, there is a specific prohibition in the T&C against entering "large numbers" of entry orders which would, combined or individually, cause margin limits to be breached. Worth checking your own T&C's? (as the resulting action by the firm can include immediate closure of the account)