To ensure it is a legitimate tax deduction - what loan documents  between me and my overseas private lender are required to satisfy the IRS.

I would assume a type of document? 

Generally, they will overnight (via Fedex or UPS) the documents to you.  

If you have a deposit you can probably find enough damage to document and keep it all. 

You as an investor, especially in a sandwich lease may choose to record your memorandum of option or other similar document to protect your interest and potential equity in the property, but you're unlikely to record the option with your tenant buyer. 

You must document,documents all damages and by all means let the son know since he is responsible mothers actions.

In the duplex basement is a working washer and dryer and it sounds like you never had a discussion about it one way or another, and whatever documentation you have doesn't explicitly mention it.Isn't it perfectly reasonable for her to think that a basement washer/dryer is included in the overall rent and there for the use of both units?

I also tell my applicants that I do everything based on what the documents say.  

A Solo 401k provider will be able to prepare IRS-approved documents for your business that will allow you to participate in the plan and get all the features these plans offer. 

In the end, everyone ends up with a bunch of pdf documents, with a random mix of some signatures digitally pressed in the pdfs, some signed & scanned, some signed & faxed.