This answer addresses "stability and cornering"
If you do motorise your trike with a hub motor, it will gain 2-5 kilograms in the middle of the wheel. This is "unsprung weight" Depending on the battery number, size and chemistry you will also add 0.5-10 kilograms of storage somewhere on the frame.
So your bike will get heavier by ~3-15 kilos. This is balanced out by the assistance provided by the motor as long as you have power, for regular straight-ahead riding.
A hub motor in a regular bike's front wheel aggravates any conditions. If you ride in the wet or on damp leaves, its more likely to slide out under you. I've never ridden a rear-hub motor, but I imagine its similar, and powering through any corners is a bad idea.
The motor is also unsprung weight, in vehicle terms that's the weight that is always pressing on the road with no suspension. Increased weight makes jumps much harder, and increases the impact of anything like potholes and road debris.
A test you can do is to get some weights that are about as heavy as the kit you want to fit, and got for a ride with these weights on your bike. Bags or plastic bottles of water might be safest. This will show you about how it will feel if the battery is flat.
Personal story - I had an electrified MTB for a while, and it was capable of 40+ km/h with pedalling. I had multiple instances of cars seeing me in the distance and pulling out without anticipating my speed.
Now I have a road bike and can get to similar speeds without the boost, and because I look faster then cars have been less likely to pull out.
I also found it was too easy to take corners at speed. Several times I turned 90 degrees and found myself halfway across the car lane, because I was riding to preserve my momentum. Once I was clear on the wrong side of the road, and fortunately only good luck meant no cars were oncoming.
Summary - ebikes are a gateway drug into riding your bike more. They're a sop to the conscience for those who want to but can't quite.... as such they do have a place in cycling.
Changing the wheel size is well covered in other questions, like:
Short summery is "maybe" if it fits without rubbing, but brakes may be an issue. Geometry of the bike is also changed in various ways. I'd expect your forks to be too short for a larger wheel, so you're up for replacing them too, if the trike has forks.