1. I think that you will just have to explain your situation to them. To be honest I'm not sure if it would hinder your chances, it may well do.
2. Not really sure about this. My AS level grades were terrible and those are the ones I applied with. In the end I was able to get my grades up at A level by 1 or 2 grades higher.
3. Different clinics will allow you to do different things, the one I went to I was only allowed to watch but other placements I went on I was allowed to do post mortems.
4. Lambing is basically helping sheep give birth. Pretty much pulling the lambs out.
5. It may be hard getting w/e here because you'd need to find a place to stay but all I can say is just write loads of letters and send loads of e-mails/call, look into places to stay. I mean you can also try to get a placement at a slaughter house too.