The house has had continued use as a duplex so I could purchase and leave as is but I could not go to a triplex as it was not a permitted use under the newer or older zoning...

Now they will consider less than two years with some evidence that the business will/continue to be successful i.e. you're in the same industry you were in previously, you bought a thriving business with an existing cutomer base etc..

I caught another podcast one day that featured Charles Roberts, I really enjoyed it and Charles recommended "Set For Life" and oh WOW!!!!!!

Typically, the same applies to LLCs where you and your spouse are sole members.Actual Lenders Title Insurance states;CONTINUATION OF INSURANCEThe coverage of this policy shall continue in force as of Date of Policy in favor of an Insured after acquisition of the Title by an Insured or after conveyance by an Insured, but only so long as the Insured retains an estate or interest in the Land, or holds an obligation secured by a purchase money Mortgage given by a purchaser from the Insured, or only so long as the Insured shall have liability by reason of warranties in any transfer or conveyance of the Title.

Should I continue with existing tenant.

My current properties cash flow nicely but I have little room for error due to continual reinvesting.

No different than people accepting poor service at a place of business by continuing to patronize it and creating a sense of norm around accepting poor service.

If so, what information would you give them to make them see things for the business reality of the situation, which is that they're realizing an economic loss every month they continue owning the house?

It may be some time to get all the parts in place.In the meantime, you should continue to monitor your holding and rebalance or adjust investments as appropriate. 

Additionally I would tell them that since they are good tenants they can stay at your new rental rate, what ever your usually annual increase is, and that they will continue on a M2M since they are such great tenants.