Gearbox oil won't do f**k all, unless it can re-weld a tooth onto a gear cog. The crunch will be the synchromesh going, and the bump sounds like one (or a few) teeth are missing. As for the jumping out of reverse, well it's safe to say it is buggered.
I know you are getting these done by the local scrappy, but tbh i would cut my loses and get the money back. There are plenty of people around that can help, and getting a decent box isn't too hard. If you can go without the car for a week, source a box, and get some help fitting it then you should be sorted.