I'm up to 10 doors (including personal residence) and have six more under contract.

Personally, it's not for me. 

Does cousin B have the cash laying around to pay all the taxes on his sale after using all his sale’s cash for this deal (to avoid taxes he must use all cash.) if the IRS denies his exchange and demands the taxes be paid?

If the IRA fails to pay property taxes or insurance bills - you are then personally responsible, which would be a violation of the IRS rules. 

We paid for the material.

I was willing and did lose what I paid for the appraisal and valuable time of submitting so much paperwork, but I think it was worth it!

Our personal  house had previous work done but the piers were poorly done, and we noticed dry wall cracks and tile floor "separation"  about 2 years back.   

You will need to allocate the expenses between the personal and rental portions of the house.

Here is why there are no responses to your question: it's tricky.Prior to the 2018 tax reform, there was a deduction for personal moving expenses.

Personally I’m having a hard time coming up with Cons, and that the Pros greatly outweigh them.