There's about $50K in repairs that should be the seller's responsibility, so now we have to decide if we want to ask for that at closing, or if we'd rather not have a retraded deal attached to us, and just bump the renovation budget from $1.34M to $1.39M.

Here they have just as much value as stick built if they are attached to land by themselves. 

The detached garage in the far back would be perfect for an ADU, but how do I determine the ARV of the property if I want to flip it with an ADU attached?

If the numbers work, you can probably have cash in hand in less than 3-weeks.I would be happy to walk you through the process and answer any questions, no strings attached. 

And, the fact that the sold price isn't attached to the deed isn't affected by what may or may not be contained in the MLS.

I did purchase an H house through foreclosure that has a mother in law unit attached.

I would never give up anything up front unless it is for simple moving costs (under $1K post move), if beyond, make an additional agreement that you can attach to retail sale of home.Seller will have to wait for his $ just like you do.$430,000 (ARV)-$310,000 (Balance could be plus or minus several thousand)-$20,000- Holding Costs (insurance, utilities, PITI, etc) about 6 months...

*judgments expire, not all judgments attach, etc.

what if the rental has a loan attached to it?

At or after the auction you need to do a title search and understand what other liens and encumberances might be attached to the property.