Hi Ceri,
1, carrot fly no idea but plant onion family next to carrots,
2, Yes that works lovely I use it.
3, cannot see why not strong enough dont over do the manure bucket full should be fine.
4 Tied around downpipe should work.
5. If used double should work cheeper than fleece,
what you save you can spend more on seeds and plants, or buy a load of manure for next year.
and dont forget the shed window you dont want everybody to see what is in the shed.