Thanks for all the tips, everyone! I appreciate it.
Since I posted this thread, I've managed to discourage him from jumping on anyone when they come in the front door by putting him on a tight leash, giving the sit command, and standing with him in the entryway while the boyfriend answers the door. So far, this has worked the dozen or so times we've had company. He did, however, whine and cry and try to take off after them once they entered the house and disappeared in the living room, to which I shushed him and made him sit back down. I wait until he's calm to join everyone (who I've already informed need to ignore him as he goes through his excited dance) and slowly but surely he's managed to get the message that the more he fusses and whines and begs for attention the more we'll ignore him.
As for jumping on me when I come in the house... He's definitely gotten better about it. The last time he "slipped" was about a week and a half ago. Anytime I leave for long periods of time I, of course, crate him, and he's gotten better about not jumping on me after I let him out, but we're working on it.
Jumping on people while out on our walks... Still a work in progress. I've now gotten in the habit of turning and walking the other way. I've tried the L.A.M. method, and he'll look at me for 2 seconds before his attention becomes solely focused on the stranger near us.
We had an incident a week or so ago where a man in dark clothes with his hood pulled up came up behind me by the mailbox and Zimmer lunged at him - I don't know if it was quick reflexes on my part with the leash, or if he thought better of his decision, but he didn't bite the guy, only managed to scare him off. Ever since, he's been associating people with their hoods pulled up as a "threat" which deeply concerns me. We live in an area with a lot of high school kids who walk around with their hoods up, so I've started walking him earlier to avoid them. Another matter which is being worked on by the trainer and myself.
Jumping on people as a "greeting" while on a walk is, as I said, a work in progress. We just haven't found a method that works, short of turning and walking in the opposite direction.