Not the OP, but I'll see if I can help. We did cavetubing.bz a couple of years ago and had a great time; here are my memories.
1. We met the guide just outside the cruise terminal. We were there early, as per the instructions, but had to wait some time for another group. Fortunately, we were in an air-conditioned van and the guides were great.
2. Our group had 7: 4 adults and 3 kids. I think it was $50 per person at that time for the adults and cheaper for the kids.
3. It's quite a drive to get to there, maybe an hour or so. Total tour takes about 5 hours, I think. You start with the cave-tubing, which is awesome, and then we went to a small pavilion where they had lunch (stewed chicken and rice with drinks and rum punch, if desired) and a small shop. Though I never felt rushed, we ended up being a little behind, so the driver had to hustle to get us back on time. But we made it time to do a little shopping and grab a drink before last tender.
4. We absolutely felt safe the entire time. The area where you get set up to go into the caves is large, with many tour busses and groups. Staff is very helpful.
5. As long as the senior is fairly mobile, yes. You do have to walk a bit to get to the river, but they'll carry the tubes and highlight interesting plants and stuff along the way. I don't recall it being a particularly difficult/treacherous walk. There's just a nice path through.
