Is it possilbe to cancel an award ticket now and pay the fee next week to have the miles redeposited?

Otherwise, DL doesn't seem to be very aggressive about searching for and cancelling conflicting reservations, but duplicate reservations might be a different matter. Also, if DL were to cancel one or both tickets, what would happen to the miles?

The OP needs to evaluate the risk and do the cost benefit analysis.

It might help to try some dummy searches for multiple award tickets on the desired flight to see whether DL is now offering one or multiple tickets (up to nine will show and can be booked on delta.dumb, but nine award tickets at a reasonable price in miles would be extremely rare) at the lower price in miles. OTOH, for a vacation destination, one award ticket might be less desirable than two or more, so someone might be less likely to grab the award ticket if only one is available for reasonable miles in the meantime according to this logic. OP might also want to consider the specific dates, which seem to be after spring break but might involve the Easter holidays or some other very high season travel period to Mexican beaches.