1. I wouldn't risk 1h 15m on separate tickets. It doesn't actually matter whether you have to "get out and back in" because you will still have to clear passport control and security. Either of those could be slow due to the additional COVID checks. You also need to be sure to have your MAD-VIE boarding pass in hand/phone before departing MEX.

2. AFAIK you are allowed to connect through MAD to a final destination. However at any stage somebody might decide that you aren't connecting and are actually trying to sneak into Spain with a throwaway separate ticket. Especially if you don't have an Austrian passport and are showing a residency permit to people who might not be familiar with Austrian documents.

Of course what you are planning might work. But during COVID you should stick to the absolute basics - if it can't be ticketed as a single ticket award, don't do it...