Maybe members could get an exclusive one that says Big Pocketeer.

To qualify for the Section 121 exclusion, it must have been your principal residence for 2 out of the past 5 years. 

Both the House and Senate tax reform proposals would change the holding period requirement for the Section 121 capital gain exclusion on the sale  of a primary residence from 2 out of the past 5 years to 5 out of the past 8 years. 

I now use exclusively quartz for countertops.

You should also try posting your listings to Craigslist, I know @James Carlson has success listing his corporate rentals pretty much exclusively through CL and gets significantly higher rent vs. long term rental. 

That would obviously cost you a lot more than your tax savings, and you would have to trust your 19 year old to be responsible with a huge sum of money.But to your actual question, your son would have to meet the 2 year ownership requirement in order to qualify for the capital gains exclusion on his portion of the house sale. 

If this remains the case, what are her or her adult son's options if this remains the situation after his passing?

For small stuff, you're dealing almost exclusively with local banks, so you either need to find a local broker who knows who will deals with OOS investors, or you need to get on the phone and start calling the banks directly.

Does it still qualify for the capital gains tax exclusion or will one year be considered non qualifying use?

No taxes will be incurred as long as the property is less than the parents' remaining lifetime exclusion amount.