Construction methods vary from place to place and even time period to time period.

I personally like the slow acclimation period as a result of house hacking, you get to gain landlord experience while having your mortgage paid for. 

You can try some local banks that hold their own mortgages on their books to get around the seasoning period.

You should do that for an extended period of time before purchasing real estate.

Seems that we are in a growing pain period in which new prices are adjusting themselves out through trial and error.

The problem with HELOC is its variable and raise quickly in an inflationary period as we may be going into.

So without property management I make $1850 a month cash flow for 7 hours of time but my flip will go on the market in 2 weeks and profit $80,000 for a holding period of 1 year after all costs.

Do I need to hold Unit B for a certain period of time before doing a 1031 exchange?

You can walk away from the deal before end of the due diligence period regardless of the reason, but you will lose the money.  

Don't ever offer more then the numbers allow.Search the average days on market in your area and after that period has passed contact him to see if he would reconsider your offer.