Burned on crust on stove top grates, paint, etc.

I just moved into a new house (new for me, that is), and we spent the other day cleaning (last tenants didn't take very good care of the place and we wanted to scrub it really well). There's a lot of very strange messes around here, but the one bothering me the most are the stove grates. Every inch of them are covered in about a half inch of crusty burnt on grease. I sprayed them with oven cleaner, bleach, everything. I have no clue what will get them clean. The inside of the oven is the same way, every wall is covered in crust. I REALLY don't want to have to get new grates and I'd like to not have the oven untouchable. I read that I could let them sit in ammonia, but the crust is so bad I don't even know if it would make a difference.

Also as a side note for those who have lived in rented places with the weird corners and such with impenetrable dirt.. HOW DO YOU GET RID OF IT? We also have a lot of the tiles and doorknobs covered in paint from the painters being lazy. Is there any way to get it off? Already tried paint thinner.. to no avail. As well as globs of dried grout on the tiles and smeared everywhere?

Thanks so much!