In my experience portfolio loans are usually ARM's, or a hybrid such as a 5/1, and yes I think they are generally for multiple properties, 5 minimum typically but again there's a lot of variation within the category and lenders can create their own underwriting guidelines so there's no standard "portfolio loan" option.

Prefab homes (not mobile homes) are usually built to the same standard as traditional homes, they are just built offsite and assembled onsite.

If  not using an agent then a real estate attorney is a good idea especially in the beginning, they can draft your PSA based on you LOI or your specific needs, they also should have a good understanding of what the standard practices in your area are and what needs to be in the contract in you locality.  

2)For Option A, which assumes standard conforming Fannie/Freddie loans,would it be difficult to quality for 4 residential mortgages within a12-month time frame?

You can start by connecting with a local REI realtor and joining groups on Facebook to see who folks are using and what strategies they're using in order to get into the market.Vet your boots-on-the-ground company by ensuring their standards for sub-contractors/GC's include licensed and insured tradesman - get references and proof of the type of work/finishes they provide.

Some deals take longer then others and you have to keep working the lead.And again keep grinding and keep putting offers in, don't however lower your standard to get the deal done. 

For example, in CA it's standard for buyer and seller to each pay their own escrow fee.

I have switched GCs and the newest one is making me front the cost of his general liability and workers' comp because he said this was an industry standard and it would be very worrisome if a GC is not passing on this insurance cost to me as the owner of the property. 

But then the property also wouldn't pass FHA either, because the FHA appraisal has higher standards than VA and Conventional.Is it really every MLS listing in your area? 

One is that it will follow the standard course colds/flus do and be done in a few months.