two questions
1. do I have to link my btc address before the snapshot?
2. can I link multiple btc addresses or do I have to consolidate in one address?
1. You can link anytime you want, but if you want to be a part of the distribution, you have to link prior to it happening. Byteball isn't able to know that you 'intend' to link in the future.

2. Yes, you can link as many BTC addresses as you want.
So, you can link 10 BTC addys to a Byteball account, or 10 BTC addys to 10 diff byteball addys.
What you can't do it linke 1 BTC address to 2 byteball addresses without an issue.