Down sizing tire size, upgrading load range, any thoughts?

Hello,
We are new to this great forum. We recently bought a 2004 2619 that came with ST215/75R14 Marathon radials. After reading about all the tire issues especially regarding 14" Marathon blowouts I decided to replace the OEM tires with either Kuhmo 857s, Maxxis, or Cooper load range D tires, plus the tires are 5 years old now. The present wheel-well clearance on top feels like it's less than 2". The increased load range per tire with 195/QR14 Ds or equivalent would be a min. of 200 lbs, and they should be a little narrower and not dent the sidewalls from the spare tire holder.
Is there a problem with running smaller height tires? I can't see why not unless I didn't replace the spare also, even then would it hurt to run two different size tires until the flat tire was repaired?

While checking the date code on the spare I checked the valve stem. I barely pushed it and it ripped open. Thanks to this forum I would never have thought about checking it. One of the other two stems is also rotten but not to such an extreme. We will have metal stems installed with the new tires.

Lastly, I think we are going to order the Doran 360RV TMS. I spoke to a Doran rep and he said they expect the sensor batteries to last from 5-7 years. Replacement sensors now cost 29.95. I'm trying to decide if we should spend an extra $30 for a sensor for the spare or not be lazy and do some crawling before each trip. Gary