We were told by several people that the decedent had no heirs.  

No it doesn't and I'm sorry for anyone like the OP or myself that has to deal with it.

There is absolutely no way we are fitting more than 6-7 guests in the property, so I'm guessing your home would be about the same.

Just doubled your earnings and have no debt.

While I was looking at syndicating deals a few years ago, I'm no longer sure if that is viable or even the best strategy for me?

that seems more vacation like.We are renting 8 to 9 rooms this house is very big it’s 7,500 sq ft with a finished basement so lots of room to fill we do plan to do some STR in between and no other listings offer these amenities so we hope to stand out

You can probably get in two at a time and still have a safe amount of capital at 500k.But if you're saying that use cash and sink every last dollar you have into a single project, I'd say no, do not do that. 

Since then the offer has expired listing agent has ghosted me with no response. 

No update of common area finishes although clean and well maintained.

In terms of your grandfather living there as long as he's not the owner having a dwelling fire/rental property insurance policy is appropriate.As @Greg Scott pointed out he should have a renters insurance policy whether or not he's actually paying rent.In terms of liability coverage, that depends on what you have to protect so there's no one-size fits all approach.