How can I sell it with seller financing if the bank already has a note on the property. 

In-person local meetups are the best and they for sure have some near you.

Can I use my personal credit cards for business expenses and somehow expense it to my LLC bank account?

Piggy bank.

He has even come to my personal house to pick up money and some things I ordered from Amazon to be installed in the property, and fronted some of his own money to keep things moving while I wait for the draw from my lender to clear.

Is this market accessible for the average investor in the UK, or is it exclusively banks and hedge funds who buy/sell these sorts of instruments?

I personally review the lease section by section before they sign it.

On a positive there are 15-20-30 year amortizations and increasing income qualifications including instant approval and AVM (no appraisal requirements) for very clean applicants, and creative documentation for self-employed borrowers or investors such as bank statements or even DSCR (usually up to 75%LTV) for investment properties. 

There are many other ways to qualify besides DSCR, like bank statement averages or even P&L statements that can offer lower down payments..check in with @Joseph Chiofalo he's a 20+ year mortgage colleague and can help with structuring and options.

Not sure how Las Vegas operates but when I get a code violation of any sort I get certified mail delivered to my personal house and multiple other regular mail attempts.