MCT is set by AS and represents its willingness to accept the risk of a misconnect. Nothing more. Once upon a time, air carriers allowed passengers to execute a waiver of MCT, meaning that if you misconnected, you were on your own and had to buy a new ticket. No carrier in the US does this any longer.
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It is correct that AS can't force OP to accept the rebook, but at some point, it will auto-cancel and issue a refund.

OP should either accept the later confirmed flight and then standby for the earlier flight. (He could ask AS to leave a PNR note waiving the fee if need be) or take a refund and purchase two separate tickets. In the latter case, MCT does not apply and it is OP's problem if his inbound flight is delayed, he gets stuck at CBP, the checkpoint or anything else goes wrong.

If OP is willing to take all the risks, AS could care less.