Do not rely on it for legal advice.

And if I were in your shoes, I would want to be a lien holder because of the legal ramifications that can be enforced as one. 

I doubt you have any legal recourse over the roof especially since you’ve owned it for 3 years.

Maybe this is something that can be written into the lease (or maybe it's not legal in a rent controlled area).This is will probably be a non-commercial multi unit (2-4 units).Thanks!

What are my risks of buying a property which was sold at a foreclosure auction to one party (who may have made immediate improvements) and then the property was taken by a party which had purchased/exercised  redemption rights. 

I would get actual legal counsel either way.

A shared fence is owned by both parties if that fence sits on the property line.

What's the easiest way for me to collect a finder's fee/referral fee for helping to connect these parties? 

If you can come to terms with a Seller, you definitely want to engage either a title company and/or a real estate lawyer to ensure the transaction is executed correctly, the deed, deed of trust and promissory note are all drawn up correctly and filed so that both parties are protected

What steps am I legally required to go through?