1. You don’t need to set the amps. Don’t you think Tesla has worked this out for the hundreds of thousands of cars it’s sold to people who don’t know what an amp is? Using the UMC the car automatically sets the amps at 80% of the outlet type. For example, plugging into a 14-30 dryer outlet with the appropriate UMC adapter it sets the charge at 24A. You don’t have to do anything or think about it. It’s made to be idiot proof. With the Gen 2 UMC it has a max of 32A for 50A circuits, not 40A. If you’re manually having to turn the amps down for some reason, you would do that in the car while you’re plugging in, not remotely from an app.
2. You’re thinking of supercharging, not AC charging. The charging rate plugged into an outlet is constant until the last few percent. Charging to 100% before a trip isn’t hard— you will know how many miles/ hr your car charges at when at home, and count backwards. Don’t focus obsessively on time at 100%. A few hours is fine, just don’t keep it there for days. Or what most people do is charge to 90% as usual the night before, and use the Teska app to start charging to 100% when they wake up and are getting ready to leave. It takes about 1 1/2 hours to go from 90% to 100% when charging at 32A. If you have 98% or 99% when it’s time to leave, no big deal. (If 1 or 2% are going to matter, you’re cutting it too close anyway).