I can't speak to the negative equity situation. While Tesla does trade ins, in my experience the value they offer isn't competitive, but I think this depends on a case by case basis. I have a '17 Leaf and they offered me $12k for it. Carvana (which is US only, I believe) offered me $14k. Based on some research, I think I can get around $17-$18k selling it myself, so that is what I'm going to do once my M3 delivery date is assigned. I'd think you'd want to avoid getting even less than the market value of your car if you were in a negative situation. Unless you have incentives that are expiring (the reason I ordered the M3 rather than waiting a bit, given how new the Leaf is), you might want to hold off for a bit and try to pay down your existing loan to the extent possible this year.
Regarding charging, I think it depends how you use the car. I don't have at-home charging with my Leaf, and I won't be getting it with the Tesla. I live in a condo in a city and while I could install charging at my parking space (I'm on the board and the other board members are pretty permissive), I was quoted at over $4k due to the need for an extensive underground wiring run. My hope is eventually other owners will want charging and we can split the cost a few years.
For now, there's a public L2 charger a few blocks away from me (Chargepoint) and I use that to charge my Leaf and will do the same for the M3, plus I take liberal advantage of destination charging opportunities. You can certainly depend on the Supercharging network, but the rates for Superchargers seem to be set at a premium above market rates, and I'd hate to constantly be supercharging my battery. As a result, I'd be hesitant if I didn't have easy access to L2 charging, but I'm sure others wouldn't have qualms existing solely off superchargers (and your situation is one the reasons that Tesla is putting more superchargers in urban centers). A Tesla isn't a cheap vehicle and I would make sure the economics make sense if you're buying all your power off a Supercharger. What I'd suggest is look to see if there are good L2 charging opportunities in your neighborhood or at/near your workplace. Plugshare.com is a good way to research this.
Now, this all comes with the caveat that I don't drive a lot. Any local trips where I'm not hauling lots of stuff are done by transit or bike. I don't drive to work, which is a 10 minute metro ride away (if I decided to, parking would set me back over $200 a month!). The case could easily be made I don't need a car at all (I lived without one for many years, including in a smaller, less transit oriented city than I am in now), but I bought the Leaf to experience the EV revolution at its infancy, and its proved very helpful in operating a side business and making trips outside of the city. My M3 will largely be for weekend trips and regional road trips. It will sit parked for most of the week. If I was dependent on a vehicle to get my to work, I might feel differently. If I considered a gas vehicle as on par with owning an EV, I might feel differently, but I'd just go back to car sharing before I bought the former. But for me, I have absolutely no regrets being an EV owner without on site charging. Yes, it is a little bit of a hassle, but I don't have a situation where driving needs to be the most convenient option for my day to day needs.
This is just my two cents and experience. Some will tell you that home charging is a hard and fast requirement, but I'm not in the camp. But as you can see, I'm in a slightly different situation than most people who are daily drivers and dependent on a car as a primary means of day to day mobility.