Aligners Removed for 12+ Hours - Can I Resume Normal Usage?

12 hours of leaving them out LIKELY won't be a big deal, but might.  I would suggest to place the current aligner in and evaluate in 24 hours.  If it still does not fit or feel as it did, then either go back to one previous aligner or see your treating dentist/orthodontist.

Don't make this a habit. However, I would wear that aligner an extra two days just to be sure you haven't lost some tooth movement

This amount of time may not be a problem if you put the current tray in and wear it consistently going forward.  However, sometimes a tooth can slip back further than a particular tray can handle if it's out for 12 hours.  To be certain about what to do you might want to visit your orthodontist and ask if all teeth are tracking properly in the current tray.  One thing for sure: if you leave the trays out every day for 12 hours the Invisalign treatment will probably not work.  Not wearing the trays consistently for 22 hours a day is most common reason for treatment failure with Invisalign.

Should not be a problem..maybe a little tighter than normal....worse case...might need to add one extra day to the normal 2 weeks

For that short a period of time, just continue with the same set of trays as if you had missed no time at all. If several days were missed, it may be necessary to return to the previous trays.