Their explanation is that the banking service they use (Thread Bank) and their agreement with Stripe (payments) leaves them with no choice but refund if such a request is made, even though they have records of the tenant authorizing payments via their own platform.

Anyone have an experience and/or advice on how to get this accomplished?   

Readers are advised to seek professional advice.

Advice on Layout/Design: For those who have added ADUs, do you recommend standalone units, garage conversions, or a combination of both?

Does anyone have experience/advice on investing in Killeen?

I'm absolutely clueless, and am looking for anyone who is willing to share their experiences, advice, or point me towards resources that could help in understanding the market better.

If you are already working with an Agent and you trust your Agent then you should trust their recommendation and advice.

I would appreciate any advice as this would be my first quadplex purchase.  

@Mark Thiel This is not legal advice but I think most LLCs are overrated unless you are required to have them for lending like with many DSCR loans.

Be prepared to demonstrate what you've learned and how hard you will work to put their advice to work.