I take it if they have been scanned your lambing early? What breed did you go for? Have they lambed before?
1) Yes a polytunnel is a good idea, I know quite a few people who use them.
2) yes to a degree they will eat it down but I find breed largely affects how far they go down, My ewes will get out on long grass to find short new growth.
1) I would think its good!
2) 2 ewes would only need about 1/2 acre but it does depend on your grass types etc. And if they are out on grass all winter then they shouldn't need feeding unless there is snow on the ground (its just nicer to give them some food) I wouldn't feed them meal though, I'd feed ewe nuts at 18% protein.
3) Cant tell you as I don't have goats but it'd think you'd would be ok if you introduce them correctly.
4) The chances of one of your 2 ewes rejecting is very very low. If they have lambed before and not rejected then i'd think you would be fine. Best thing you can do is just leave them alone when they are first born as if you get involved sometimes the ewe will turn on it. I had this problem with my first lambing, it was a shearling and I moved them too quickly into a pen and she knocked the living daylights out of him. She took it in the end but after 24hours.