When I do, rather than fixing it up (built in 1948), I’d like to knockdown and rebuild, but split the folio to convert it into 2 townhouses, then sell 1 townhouse to pay off the construction costs and have a new house paid for.

If the manufactured home has been converted to real property, then there are more financing options available to you and the your end buyer.  

New laws allow you to convert duplex garage to a 1 bed ADU and the SFR garage to a 1 bed ADU, in my opinion theres no better strategy to get started in Cali.

As a current basement dweller and someone who slept in a converted living room for years in the NOVA area, I have somewhat of an idea about room rents in the area, but I've realized that there's a lot of discrepancy in advertised rents on Craigslist without a way for me to have a reliable indicator of how much rent my property/room configuration could get to calculate if house-hacking would be beneficial.

If you aren't able to convert the leads you have, your kink in the system is likely as @Joe Villeneuve said.

That's when through shuffling around on BP I was introduced to the idea of a solo 401k as a potential option to roll or convert my prospect Roth IRA account into.

I'm also interested in purchasing another property - either multifamily or another vacation rental (which will be classified as an investment property from the start) - will converting our soon-to-be current property into a vacation rental impact that process in any way?

By right you would be able to convert that empty ground floor space to 15 apartments!! 

The 1031 deferred the capital gains taxes, and then converting it back to a homestead enabled him to use the $500,000 (for married filing jointly) exemption to capital gains for personal residence.

I think the home will need about $6k-8k as I would want to add a wall to convert it to a 4 bedroom (which I did to my home) and paint and/or upgrade the flooring.