Let's take the last question first about whether to use miles or not. Of course it all depends on your perspective of value and circumstances, but the general idea is to get the most value for those miles. So, using miles for a flight or upgrade that costs $900 is a much better value than using the same amount of miles for a flight/upgrade that costs $400. However, if you are unlikely to ever take that $900 flight (or maybe the miles are going to expire) then using them on a flight you will take definitely makes sense.

It does seem to be correct that only AA has a non-stop flight between MIA and MID.
BTW: for the week of Apr 17 - 23 those AA non-stops range from $501 to $679. If you can hold off to the next week (after Easter) then AA's non-stops range from $340 to $511.

Delta has flights to MIA to MID but they require either one or two stops and range from a low of $351 the week after Easter to $1,450 the week of Semana Santa.

Yes, both United and AeroMexico offer itineraries having 1 stop or 2 stops. Avianca also has an itinerary but it has at least 2 stops and starts at $2,700+.