I personally would use the money to flip in lower priced areas where it is easier to find deals, some local-ish areas I think are good include: Rialto, Yucca Valley, Bishop.

Disadvantages - more risk only one income stream when you go vacant, cap ex cost more because it’s not shared by two or more income streams, generally lower cash flow than a multi family. 

In my case at least, there are permitted uses in a zone which are listed, but if the existing use of the property isn't the use you plan on even though it is permitted, then it goes to the Planning Board, a lower hurdle. 

If you can, I would always try getting owner financing first as it allows you to get easier terms/lower down payments than having to pull your own money out for your purchases.

Doing my first cash out refi. 3.875% 30yr lower fees.Question how long should underwriting take?

Although their purchases are usually lower, so yes, we should see an impact, but I think it will be more due to uncertainty and not actually being denied. 

I got a pretty good rate (much lower than typical HML).

You have negative cash flow (which is normal while House Hacking).To improve the deal you would need to have a lower purchase price and/or be able to increase rents.

Correct me if I understood your suggestion wrong .. 1) Refinance the TH, which I currently reside in (primary home with 4.75) to lower the interest rate. 2 )  Do HELOC on the first SFH  ... here when I do HELOC, am I able to switch it to 30 year amortization same time (I need to refinance this place as well to increase the cash flow) or I have to do cash out refinance instead of HELOC?

I would approach that from the standpoint of the more I put down, the lower the monthly payment would be since it reduces the amount financed.