I’ve been following the forum to build my knowledge as I prepare to purchase my first investment property.

Some are more aggressive in pushing to conforming uses and some are more friendly. 

Its not being followed by the other two mortgage insurers as far as I know.

The high demand that those two counties have pushes price points up on the low end properties so that they simply are too close in price to the high end properties making flipping more difficult.PG county however, (along with DC) is very easy to flip in. 

The cause of a bankruptcy is irrelevant, medical or otherwise, what is important is the length of time since the bankruptcy and there record since.Simply because it was caused by medical does not change the fact that following a bankruptcy a individual is financially fragile and a very high risk.

It comes down to the mortgage banker knowing their guidelines and the bank following guidelines.

How far can I push in a professional way to ask an agent to prove they have multiple offers?

I wouldn't nag them about it.There are a few ways this can go bad for your relationship with them if you keep pushing.

If not you will end up as just another faith investor.With the value pushing up into the $600K range your rents are far too low and need to be ajusted upward or as I stated you will need to sell to maintaine the returns on your equity.Unfortunatly the majority of CA seemes to be a losing investment market for cash flow so inevitably you will need to either get out or heavily supliment your tenants rent out of your own pocket.

You need to make sure that they are an active participant in your program.As examples:if you say you are looking to find off market properties, that would mean that the agent would need to call around neighborhoods looking for potential sellers and follow up vacant home leads.