I have 2 outstanding questions about mailman configuration options:

1. Does mailman have a configuration setting such that everyone who is subscribed to a list can post unmoderated, but email from a non-subscriber will be sent to a moderator for approval?

Yes or No?

Yes

To let some non-subscribers post without moderation (a practice that I personally feel has dubious value, since their question might be answered in a post to the list, perhaps in response to someone else asking the same question, but they won't see it because they aren't subscribed), how about implementing simple filter so that if a non-subscriber's post mentions one or more keywords, let it through to the list unmoderated, otherwise it goes to the moderator?

Is this currently available, or not?

No (at least not without hacking SpamDetect.py or something similar)

2. Is [content filtering on the headers or body, to block messages that are in reply to an entire digest] possible with mailman? Before I move my majordomo list to a new server with new mailing list software (mailman is the software I'm considering, obviously), it is a requirement that I can configure that software to do the same things I can currently easily do with majordomo. I don't want to waste my time trying to futz around with installing and configuring mailman before I learn that it won't let me do some of the critical things I need it to do.

Is this available, or not?

Yes. Automagically, the messages get held for moderation. With hacking of SpamDetect.py (in this case, just adding entries to a table), the messages can be rejected without commentary or further action.