Whats going on Adriel,If your contract is assignable then you simply just need to have an assignment agreement with a new buyer.You do not have to have a seller sign off on this since the assignment language is already included in the original contract - the seller would just see a new buyers name on the closing documents.

And once it's been documented it has to be disclosed at the next resale.My main questions are - If I do want it tested for lead and asbestos in the inspection, would that be something my mortgage company would require to be fixed to close the sale?

Once all is agreed upon and signed between your team (you and your attorney) and their team (the seller and their attorney), You will then need to turn around those documents to the lender who will go through the process of qualifying you for the loan, applying for the loan and closing on the loan. 

The siblings and her have done the following-Checked with original manufacturer of home for paperwork - Searched the home in all the places these placards could be- Contacted county register of deeds for records- Checked with local Town authorities for records- Checked with state DMVNone of these have produced a document of when it wad built or put on the property.

Just properly document the movement of the funds.

If you don't already have the Association's governing documents I would suggest going on the County Clerk's website and searching the Official Records.  

However, they have have documents reference themselves or the LLC and there is no consistency.They will normally have title and collect rent inside of the LLC.They will normally have mortgage interest and insurance in the personal name.I would be really interested to hear whether this 'comingling' would even provide protection to the investors.

I’m even willing to make the payments under the gamble of never getting it refunded, if I can get proper documentation. 

You probably want to make the lease and option two completely separate documents. 

In commercial a 30 day close in very fast and it is only possible if you have all of your systems and documentation ready to go from day 1.