1/4" of tread = 8/32" of tread. The tires when new probably had 10/32" tread. A 2/32" tread difference front/rear is ok. That said, do you really have 8/32" tread on your two best tires? You need a tire tread depth gauge to accurately measure tread depth. Eye-balling it or using a quarter really isn't going to cut it here.
Is the spare tire the same brand/size as the michelins? I bet if you remounted it on the rim it would hold air.
You can always look for a used tire if you can't afford a new pair/set.