In terms of making a unit legal, I was on a call last week with some alderman talking about the ADU program and some of the challenges.

You can program from remote location, key cards have to be done in person.

Income (W2/business) comes sustains RE, RE doesn't sustain income, at least for quite some time, unless you happen upon a unicorn property.Then buy a multifamily place, use everyone's income to get a loan on it (there are financing programs around this) and have other people help to pay your mortgage. 

 https://www.nyc.gov/site/mayorspeu/programs/tenant-rights.pa...

All the construction for the ADU was unpermitted/undocumented and the previous owners were contractors themselves.I want to apply to the Oakland Amnesty program, but one of the requirements is that they need proof of the establishment/construction and also occupation of the ADU prior to 2021.Since I just bought the home in late 2022, what is the best way to go about the proof of establishment and occupancy requirement?

Lenders that have flexibility to originate in multiple states and have a wide range of programs available provide more value. 

They have some interest only investor programs that can also enhance returns.

All the big name gurus will promise you that their program will teach you everything you need to know to be successful...and then this hit you with a MASSIVE price tag.

They are addressing this by their Roads program into Africa.  

Some people swear by his program.