I agree with the OP, do not let your emotions make you feel like you have to move forward on a deal that is no good.

OP,I don't know where you are but in So Cal, the OZ are seeing a lot of investment these days.

Well, looks like the OP has left the chat.

OP would have a better time trying a vendor take back in the US because it’s way more common and people are on the fritz.

This 1) undervalued the cost of a kitchen 2) undervalued the cost of construction that is not done at the same time as the SFH.Until there are enough comps of houses with ADUs to justify the valuation, the appraisals will be on adjustments and the adjustments will undervalue the ADU.Does this mean I am against the ADU in the case of the OP. 

The doc stamp for note is $.35 per $100 of the value of the note.Miami-Dade has an additional county tax on top of it.https://floridarevenue.com/tax...So to answer the OP question, yes you will have to pay a 0.35% doc stamp on the max value of the HELOC.

I have transitioned myself out of my business’s daily ops, he is making this impending flip his full time obligation the minute I secure the HML.

The OP did awesome with his purchase.  

Here is one we use often:https://www.homedepot.com/p/HO...OP, no to any SD deduction on this one.

Agree with you that OP's rentals most likely constitute only one trade or business, thus he has nothing to aggregate.