Migration Assistant - will this also transfer software licenses for apps I have purchased on old (to be decommissioned) MacBook Air?

That will depend on several things:

1. Developers have different software license agreements; some can be ported to another computer, some cannot (only one install or computer per license). You need to check with the developer(s).
2. Depending on how old those apps are, they may not be compatible with the latest OS and/or the developer changed from "one time pay" to a "subscription" model - again, check with developer.

Yes, it should. It migrates your User/Library folder where the info is stored.

Maybe. It depends on how the developers implemented licensing and authentication.

You may be required to re-register some applications and in rare cases, re-install them.

Your post reminded me about deactivating any app on the old computer whose license is good for only one computer before migrating. This will let the new computer be licensed.

You're welcome!