I have been here ten years, most of that on tourist visas. You assume some risk but I find it worth it. You may not be able to do unlimited border bounces (or you can, nobody knows), but if you're creative and determined you can stay here a long time on tourist visas.
Turn off the airconn. If the house is warmer the water will seem less cold. I have two showers and the water heater broke in one, I continued to use it for a couple of years before I bothered to fix.. Not hot showers is not the same as cold showers, you'll be fine.
They will likely cut off a hand if you miss a check-in, especially if you're in another country.
Check-ins are for 90 days in the country. If you leave, you are not in the country. The last I knew (things may have changed), you do check-in when you return..
I would not change your flights. You should be good to enter on visa exempt.
If/when your DTV is granted, you will need to exit/re-enter Thailand to activate the DTV.
If you have no travel plans you can go through an agent to a near land border, cross, come right back (border bounce). You can do it yourself also, if you're into adventures.