much easier to apply for non-o in your home country. There are some catch 22's doing it I in country forcing you to use an agent and then being at their mercy for future extensions....
Most likely will not be able to get insurance in Thailand for a 79yr old. If you can find it, it will be exhorbently expensive.
If your intention is NOT for her to stay long term then best to get travel insurance from your home country and then arrive for 60 day visa exempt. You can extend it another 30 days when you're heading the end of your 60.
Think long and hard about trying to invest in Thailand. Your money will make more in a simple certificate of deposit than you will here. Rental income can be very seasonal and liquidity is very low.
I think it was a marriage visa and there's plenty of rain to justify a short stay to "clean up affairs" after a marriage. Just need to be honest and open.