(in Muay Thai)In the situation where your leg is held by the opponent, is it allowed, legal, or reasonable to throw a kick with the standing leg and land in a similar fashion as you would with a cartwheel kick? Eg. kick with the standing leg, fall with the hands to the floor and bring your feet to the ground afterwards and then stand up from that.
The legality of the move has already been answered by Juann and others in the comments.
Even if it was legal, this would be a poor option. If a Muay Thai opponent grabs your leg (catches your kick), the next thing they are going to do is smash the thigh of your supporting leg. They are not going to give you a moment to start launching your own convoluted cartwheel kick.
Your best option in this scenario is to:
lean forward keeping your guard up. This closes the gap a little and sets your body position up
activate the hamstring and gluteous muscles in the held leg. These are enormously strong muscles, the opponent will either have to let go or they will also start to lean if they persist in holding on. This will also upset their rotational/twisting movement and balance if they have already launched a counter kick at you.
While I broke this down into two steps, this needs to be done smoothly in a single motion, you don't have time to stop and think. At this point you should be close enough to engage in a clinch. If your leg is still held then you can clinch and use the leverage from that and the held leg to launch the knee from your support leg. If your leg isn't held then you can still carry on attacking from this position.
Attempting such a kick is one thing, but given how easily the opponent can pull push or twist you with the leg they're holding, and all the ways in which your attempted kick might miss or make more or less contact than hoped, it's very presumptuous to assume anything about how you'll land afterwards let alone "fall with the hands to the floor and bring your feet to the ground afterwards and then stand up". You're much, much more likely to end up on your face or back, possibly with a kick or knee to add value, and I'd say it's 50/50 whether they're still holding your leg.
As for rules: "Muay Thai Rules 19: Knock Down - Definition - Any part of the body touching the floor except the feet." - so if you manage to land on one or both hands, you're the one who's technically been knocked down. The scoring rules require "correct Thai Boxing style... strike", where-as the proposed strike - landing on your hands - clearly isn't Thai Boxing style. If you were to jump up into a knee, front kick or roundhouse and land on one or both feet (and nothing but feet), that would be ok. Rules here