Small, easy to get around, beach bars, and affordable condos are still accessible (though I worry for how long and if it will end up over developed eventually).

I’m a huge fan of the multi-family model of investing and would like to eventually build up to owning apartment complexes.

Eventually, I left my SF rentals in my name and beefed-up insurance, including an umbrella. 

And that doesn't include other expenses.So the traditional SFH route may work eventually including appreciation and other factors, but probably not in 10 years because you need to refi and purchase other houses to make this angle work.Buying a 2/1 somewhere and living on one side is another option, as is renting both sides, but you'd have to identify an area where you can afford a duplex.

You have to be willing to play the long game and overpay with the intent that eventually after renovations you will take maximum market rent and turn a profit.

Eventually, I’d like to get into long-term rentals (out of state) and short-term rentals (both in and out of state).Besides real estate, I’m really into outdoor activities like surfing, mountain biking, kitesurfing, snowboarding, skateboarding, and rock climbing.

Quote from @Kendall Jackson: I’m trying to get started in real estate to provide additional income for flight school and eventually buy a home in San Diego.  

You’ll have to pay taxes on your 401k eventually.

Since the remaining tenants have still been paying, I'm not sure that I can treat the MIA tenant as absent.Let's say the remaining tenants decide to move out (probably because they will eventually have trouble paying) and I make the refund check payable to all the tenants per the rental agreement (and to protect myself against claims from the MIA tenant), but they can't cash it because they can't reach this third roommate.Any recommendations on what to do with the money or what course of action to take?

It may be a gas furnace that has a flame in the basement and that heat eventually rises through the ducts to the 2nd floor.