Rough Housing Math

Since I last posted (yesterday), a few more promising houses have been listed and received offers. Oh well. The question at hand: how can Frugal Son--even with help from Mom and Dad--pay for a house? Here's my rough math: tell me what I'm neglecting to take into account.

Frugal Son is ALREADY paying $600/month for a tiny, shared apartment. He had to find an apartment PRONTO when he got a job. Still, he's in a nice neighborhood, so he wouldn't save much by moving to another apartment.

The realtor suggested that Mr FS and I--with our good credit--take out a mortgage and then SELL THE HOUSE to Frugal Son. We can get a lower rate.

Let's say the house is $300,000. Forget the downpayment. At current rates, the cost for a 30 year mortgage would be around $1200/month. Add about $400 for property tax and insurance...and we are up to $1600.

Frugal Son is currently paying $600 for half of a 600 square foot apartment. If he rented a bedroom in his house to a friend for the same $600, he would be on the hook for $1000.

1. Frugal Son said he could swing that, if necessary.
2. Frugal Son will be making more money soon, probably. He is working on his teaching certification.
3. We're committed to gifting Frugal Son the amount we saved for his education, which we didn't use, owing to massive scholarships. So, if we gifted him the $400/month difference between his current rent and the mortgage on his new abode, we could help him for AT LEAST 100 months.

Now people had these fantasy scenarios during the housing bubble with disastrous results. So to consider possible disasters.
1. Does Frugal Son need a roommate? Yes, but it's not absolutely essential, especially if his house is under $300,000 in the example.
2. Can we help out if roommie doesn't appear or work out? Yes.
3. What if Mr FS and I lose our jobs? We ARE tenured, but I've run some firecalc* scenarios, and we can survive a frugal retirement even if we were both fired.
4. What if Frugal Son loses HIS job? Well, he can make just as much by being a substitute teacher.

What is ALL OF THE ABOVE OCCUR SIMULTANEOUSLY? We have that education fund safety net.

*Firecalc: the best retirement simulator I've ever seen. Check it out!

Am I missing anything here?