How long can we wait/should we wait to start at a new company after H1B transfer is approved? Are there any implications if I start a couple of months after receiving i797 for the transfer?
Lawyer said within 60 days is ideally expected. You will have to work with the new employer to allow you to join later than that, plus rather plan your transfer accordingly. Why would you anyway want to lose time once the transfer has happened?
From experience, not an issue at all.
Not I am aware of. You should be able to join the company anytime within its validity.