1. I guess I can apply for EB1A I-140 even with TN.
But is a visiting position good for I-485 application?
-The permanent/renewable position is required for I-485 approval.
2. Is H1 visa better than TN in terms of GC application?
-H1 is better. After I-485 filing, you can not renew TN, but no problem in H1.
3. I may get an offer as a researcher from a company. Is this position better than visiting professor for GC application (though I prefer to work in academia)?
-Depends on whether the position is permanent or renewable (mutiple times).