I'd be hunting for a used engine for ~$3k and a shop willing to do the install if you couldn't do it yourself, I'd be surprised if it was anything more than $2k to bolt it in.
It blows that they're blaming you, but understandable without evidence of past oil changes given that it's a used car. This is one of the reasons a full service history is recommended on used cars and why several here wouldn't want a second hand 86. The shitty thing is I think odds are low it was anything the prior owner did as this is a not unheard of failure.
Hopefully if Toyota doesn't drag their feet (either way) and you can get a solution worked out in a few days and be back on the road asap. This car wrecked my faith in factory warranties, if my engine blew up this weekend I wouldn't bother calling up the dealership.
