I.
1. Assumption: Our Nxt has TF, a good portion of TPS and everything is fine.
2. Now we add parallel chains with all the things we discussed so far to it. And people start building parallel chains.
3. What does this to our main chain? Technically speaking. What are the implications?
Luckily it doesn't affect the main chain at all (technically they are like having LTC and BTC on the same computer if you are running both).
The idea of putting in "checkpoint" messages would be to simply add 1 small AM per day per chain (would hardly be noticed even if we had a thousand of them).
II.
Is related to 3: Please summarize what the technical benefits for our mainchain are by having parallel blockchains (besides the point that we can 'offload' features to other chains)
It makes Nxt the ideal "platform" to experiment with new blockchains and to test new ideas without risking failure to the main chain (alts are now being used to trial ideas that might be useful to Bitcoin because they simply can't risk trialing anything with Bitcoin).