I have heard 1) That the buyer can just be added as additionally insured on my insurance. 2) That I can be added on as additionally insured on their insurance3) That I need to hold my own insurance and the buyer needs to have their own as well. 

We will be adding another deck, EV charger, and renting it as a STR.

Quote from @Justin Spear: @Jay Hinrichs yes, first question: is there market value in me clearing a house lot on rural land, putting in a driveway and getting a septic design, then selling at that level of development. second question: how much more value is there to be added by introducing a MH and selling as a turn key starter home versus partially developed land.  

Will be adding 16k of renovations to unfinished side.

Unless you have evidence the tenant caused the crack, you are out of luck.It sounds like there should be a bead of caulk between the sink and the wall to prevent water from seeping down.It also sounds like you should inspect at least once a year so problems like this don't go undiscovered for so long.

.- Are there any other endorsements / options that can be added to a standard policy which act as a substitution or supplemental for an EQ policy?

Something I should be adding into my calculator when I run numbers?

If the property is in Nashville, some things can be added to attract bachelorette parties. 

We will be adding them to our website later this week - we are pretty impressed

You still need to maintain a slope of about 1/4:12 and you'll probably be adding horizontal distance by going around the perimeter, so you just need to make sure there's still enough vertical drop to get your desired slope.