What this means for you s that there is a good chance that your existing SFH becomes rent controlled when the ADU is placed in service (I would like to see the law clarified on this as this practice goes against the rationale of the 15 year exclusion). the reason this is important is that any rent reduction places your base rent at that point for rent control purposes.  

Even if you have to pay more for the purchase think about the costs of carrying the real estate while waiting for the approvals  (debt service, property taxes, insurance, maintenance for the in-place tenant) and the additonal finance charges associated with originating an acquisiton/bridge loan and then a construciton loan at a later date. 

Compensating factors ie. weaker credit score, weaker reserves, weaker DTI2.

Also, if you setup a servicing company to manage the payments (usually like $25-35 a month), they can setup an escrow account for taxes and insurance as well and cut a check to her Trust bank account each month.

What customer service programs do you have in place?

High cost of entry here, and I have found that most local lenders require 20-30% down with 6 months of reserves for expenses in case of vacancies.

My company does full service property management as well assisting with purchases so we have a ton of great insight and knowledge we would be happy to share. 

So get a service that will make sure that there are enough funds to be transferred to paying for the seller's mortgage. 

Cannot use a HELOC for assets and cannot be used as PITI reserves.

How much in financial reserves does the HOA have?