A: You mix 1 part of this compost with 4 parts topsoil. That is 8 lbs compost with 32 lbs topsoil which gives you 40 lbs total.
A: I'm planning to use it for mine. Square Foot Gardening 'Mel's Mix' recommends more than one type of compost, so this blend looks perfect. I doubt it's enough by itself for a 4x4 raised bed, so I'll probably add other types as well.
A: I would give that a big NO for seed starting. Use coco coir and perlite until seedlings get first true leaves, then up pot into a mix with compost.
A: For seed starting, yes. For Mel's Mix, I would buy separately: manure (cow/steer/ chicken) + mushroom compost (check locally for a mushroom farm or find online) + Land & Sea + earthworm castings + composted forest product/bulk compost.