I'm excited to respond to this question. Yes, it will be difficult, but it is doable. Please see photo from today. There are a few things to consider:
1. Clearance between the side of the car seat and the rear C pillar. It's tight regardless of the carseat and the height of the driver and the door has to pretty much be opened completely which may not be possible or convenience where you park to unload your kid.
2. It will depend the height of the person sitting in front of the rear facing seat.
I'd venture to say that the driver should be more comfortable in his/her seating position than the front seat passenger. Given that but also considering that the i3's front passenger knee clearance is extensive, let's just say you're okay with the front passenger being a bit less than super comfortable and s/he is less than 5'-9". If you're the primary driver and your wife/husband/partner isn't taller than 5'-9", your kid go go behind the passenger which would be perfect. Easy to load on the curb side for places where you drive on the right side of the road and curb is therefore on the right.
This is my seating position as the driver. I'm 5'-8" and the seat is pushed forward maybe 1-2" forward than ideal. The rear facing Britax car seat is big and bulky. If you zoom into the picture, you'll see that it's close to being in the most vertical position (with the black marker in the green zone) to take up the least amount of depth in the rear cabin and therefore giving the driver more space.
3. You need to consider that the the coach doors block you in when you're parked parallel next to another car such as in a parking lot/structure. You won't be able to remove a convertible car seat at all. The logistics of the doors swinging against each other and having to open and close in a particular order mixed with the close proximity of the car next to you limiting your space just makes it impossible. So if you plan to unload while parked next to other cars in tight spaces, forget it. It's probably not worth the trouble of having to circle around for the "end space".
The thickness of the door makes it even more of a challenge to open the rear door enough to get your child out of a rear facing car seat, let alone the entire car seat itself. Don't underestimate the benefit of being able to take out the entire convertible car seat. You can take your kid in when s/he falls asleep, amongst other benefits.
If you haven't gone through the exercise yet of opening the rear door as the driver, you have to test it. The driver seat belt is on the B pillar which is built into the rear door which means you have to unbuckle yourself, come out of the car, before the rear door opens.
Note that you can't easily come out of the car on your own as a rear seated passenger. You'd have to reach for the front door from the back of the inside (assuming your arms are long enough), push the front door out and open enough to stay put, then reach blindly at the handle of the rear door, swing that out and then climb out. It's not something you'll want anyone doing, pretty much ever. Count on the front seat people always letting the rear people out, no matter what their age or ability/disability.
"The i3 is essentially a 2 door car with 2 bonus doors."
Don't compare it to a sedan. Compare it to a hatchback coupe. Think about the people who drive their BMW 4 series (not the 4 door grand coupe ones) with kids in the back. It's not convenient, but it's doable. Much easier with a Mazda 3 that has 4 normal doors to do the same thing of getting people in/out.
4. If you have a rear facing car seat, chances are you may have a larger sized stroller. Take that into consideration. Check fitment for yours. This is crucial b/c it could mean that you have to fold one of the seats down to fit your stroller and your wife/husband/partner who may normally want to sit next to the child, must sit in the front.
I personally don't have any of the issues mentioned above. I use the car mostly for commuter and I pick up my older forward facing child from school to home. Wife has a larger car that we would use primarily if 4 of us need to go somewhere. The i3 is not designed as a family car, but it's doable if you know the drawbacks and how you'll respond to them. For me it's worth it for a number of reasons. The car is just super cool to drive, the prices for second hand ones can't be beat, it's electric, the list goes on. But there are compromises. Not as many as you'd think. They say the car has the footprint of a 1-series BMW but the interior space of a 3-series. That may be perfect for city dwellers to park in a congested city.
When searching for a car, the things I looked for as my criteria were good crash test results, being short in length for easy city parallel parking, good on gas mileage for my lone commutes, workable with kids (I did want a BMW Z4 or SLK 350 but that's not happening with kids I need to haul around on occassion), under $25k, under 20k miles. I ended up with this after comparing to the Prius Prime and Tesla Model 3 but of which were over budget, the latter being more of a dream than a realistic financial acquisition.
I don't drive this setup normally. I literally just installed this second car seat behind me as the driver earlier this afternoon. If I planned to do this often with two kids, I'd swap it so that the front facing seat is behind me and the rear facing one is behind the passenger. But b/c I normally only have one child in the car (the front facing one), I'd prefer her on the passenger side to balance the car better and have better access to take her in and out from the curb side.
Don't get stuck. No what you're getting and more importantly, what you're NOT getting. Know where you're compromising and be okay with that. There needs to be a pretty compelling reason to go with a car with coach doors and the need to take the entire car seat in and out. But do realize that the car should last longer than the time you'll be going through all of that. In less than 15 months, your kid will be front facing and the whole coach door problem is lessoned to a big extreme.
If you're like me and your diaper bag isn't a monsterous thing and instead, it's a Chrome Kadet sling bag, you'd rather get a little wet than carry a golf umbrella "just in case", your water bottles for your kids aren't full size b/c you can and are willing to refill on the go b/c you like stuff as compact as possible, then you'll most likely do just fine. If you're like my wife who literally pulled a hammer out of her purse on our first date, then you're going to struggle with the car.
If you can't already tell by my long winded response, I really do want to help. Reach out. I have this thread subscribed. Bring on the questions.