I neeed your opinion

well if your getting a new motor in 6 mos and the old 1 only see's 250 miles
i dont see a problem as long as you always have the strap tight which makes it feel like a solid motor mount... i definately would'nt go racing, but i would go cruisin..
im not saying this is a good idea but under the circumstances and plans
you have, its probably the best idea out there.
welding the mount to the block would work to, and if you welded it right, you could probably get the mount back that your going to need with some grinding and drilling..
im pretty sure its called a carbide steel drill bit which is the only bit that will drill through an easy out but even that is not without complication and time. (the name may not be correct) but there is one out there that will do the job. bieng the motor is in the car and you would be working upside down
also makes it 3 times harder to do rather that flipping it upside down on an engine stand to apply the correct pressure and have the hole centered.

The broken bolts on the PAssenger side? THat's the compression side. Strapping that side will only make the condition worse.

Don't hammer the car in reverse.

Long as the second bolt is secure you shouldn't have a problem, if the mount slips it will move into the block. It shouldn't move so far as to cause any major misalignment. Like fan into radiator.

Give up on drilling the easy-out. Easy-outs, at least the quality ones, are extreamly hard. (No kidding, right?) Could tell about easy out/tap extractors but that would start a rant about virturally useless tools.

About the only way to get an easy out out is with an EDM.

Unless your absolutely set on getting another bolt in I would give it up, that pooch has been pretty well screwed. You may get lucky and have the easy out fall out.

Good luck.

EDM = Electric Discharge Machine. Think of it as reverse welding. Instead of the arc adding material in welding, the "arc" of EDM removes tiny pieces of material. EDM requires submerging the piece being work on under an electrolyte solution. Hence, it ain't going happen with an engine together, and its probably too expensive to be worth it in your case. Time to get a new block.

As MadMark pointed out, the Easy-Out is hard... actually hardened tool steel. So your drills are useless as you found out. You can try the carbide drill, but I'm gonna guess that the "carbide" drill you find at the hardware store are actually carbide coated on high-speed steel, so you'll just end up wearing off the coating. You can try to find a solid carbide drill, but it'll cost more $$ and remember that the carbide is more brittle than steel so don't shock/slam it. And technically carbide should be spun 3x faster than HSS, but whatever.

Try to heat up the Easy-Out directly with a torch. Metal has to expand, and if it can't expand in diameter because its trapped, it will expand in length. The Easy-Out should cool and shrink in the new longer, smaller diameter shape. Hopefully enough to turn the bastard. Or try it a couple of times.

Another thing I've heard is to try to weld a rod to the end of the Easy-Off, that way you turn the rod and the Easy-Rod turns too. Be carefull not to weld the rod to the block. I've heard of making a "washer" that doesn't weld and put it around the Easy-Out to protect the piece you don't want to weld.

Hopefully this helps out in some way. But I don't think you can do anything while the engine is in the car.