If this were my deal the sale price on a triplex would likely be around $900k, so if I bought the land for $150k, and my build costs were about $500k, I would expect my interest and carrying costs to be about 10-12% of the total of that, so lets say $75k, for all in expenses of about $725k. 

Listing photos were terrible, tenant vetting was lackadaisical, checks were sent to wrong addresses, tennants weren't properly vetted, charges for "mulch" and landscaping were charged but never physically carried out.

Buyer of note thinks seller still carries the insuranceWe sold a loan once to someone and a few months after we sold it the buyer of the loan called us asking for a copy of our insurance.

However, if you're running your numbers then you'll want to make sure that you're including all the interested paid for the money borrowed from your HELOC as well as the carrying costs including utilities, insurance and property taxes.

I tried to get the owner of three of the houses to carry a short note but no dice.

An individual carrying back the sale of their home (owner occupied) won't be held to higher institutional standards, RE operators.investors may be being in the business. 

You will benefit from better rates and lower cost lumber now, so that will help make up for some of the added carry and perhaps the market conditions have even improved? 

I've also seen houses there in prime locations that have been for sales for years and didn't sell for one reason or another, so building a spec house would also carry it's own risk. 

So hard money loan in my name, down payment and carrying costs covered by her but we still need to meet to hash out the details.

I think your numbers are a little high (but still very good) as you'll need to consider your carry costs during the 4 months.