Before seasoning period ends, I want to sell to my younger brother who purchases with conventional mortgage.Is there anything wrong with this idea?  

There is no requirement to use them as mentors for doing the rehab projects as described in the horror story above, but they do their own underwriting so they will be investing themselves to assure the deal makes sense and is successful to assure they get their payments and interest. 

When wholesaling vacant residential land (city lots or larger residential parcel in rural areas) how long is the due diligence period?

A truly "motivated" seller tends to not really care the amount of earnest money and you could also structure your EM due at end of your inspection period.

They will take the permit fee and send an inspector that will provide some assurance that the work was performed correctly.

A buyer agreement is the legal document that provides the agent with the assurance that he/she will get paid if they work with you to complete a real estate transaction. 

With this strategy, we knew exactly how each house was operating and we could intervene or change leadership of the house if necessary to assure operations were going as we expected. 

Rest assured @Tre DeBraga you are NOW on the right path!

Your personal loan has a 5 year balloon period.  

Despite being assured that there would be significant tax savings on my 2023 taxes, there were no cost savings or advantages to having paid $5,000 for strategy in that tax year.