I have tanned a lot of rabbit skins with alum and salt. Your making more work than necessary.
1 kg alum, 1 kg salt, 15L water.
Skin rabbits and put straight in freezer till you have around 15-20. Don't salt or flesh them.
Mix your solution, put them in for 2-3 days, stir twice a day or more if possible.
Pull them out and rinse them. Beginning from the backside peel membrane from skin. Should be quite easy.
Keep old solution and add half as much salt and alum.
Put skins back in for 10-14 days, stirring whenever you can.
Pull skins out, rinse them, hang them over the bath to drip dry usin