Given the expense and complexity of a higher power charger ... crazy question, do you really need one?
If you could fix the grounding issue with the garage 120V outlet, could you get by with L1? I've actually been using L1 almost exclusively for the last 2 months (I have an L2 in the garage, but it's hard to get to ... long story)
I do have a quick charger in the neighborhood if I outstrip my L1 charging ability. But at ~30 miles per day on average, I was fine with L1 charging.
(Nissan does seem to recommend that the L1 charger be used only on a 120V circuit which has been validated to be able to handle the continuous load, but that might be a lot cheaper than the higher power install you're looking at).
Living in MN you might have been able to get by this summer with L1 only......but winter is an entirely different animal for an EV
I also got by all summer and through December but by January, I cried uncle and got my first L2. The thing is even with only a 30 mile commute, in the bitter cold and snow you may only see 35-40 miles range, and thats with a 12 bar battery. 30 miles would be doable in all conditions, assuming you have at least a 11 or 12 bar battery but the problem becomes when to charge. To completely fill your battery you'd need to be plugged in 20+hrs/day, doesn't leave a lot of time to drive and you'd have to have access to a quality 120v outlet almost all the time.
My first L2 was a 40a Juicebox and it made all the difference in the world. No longer did I have to constantly look for places to plug in, in a little more than 3hrs I could go from almost empty to full! Another big plus with L2 is pre-heating your car. With L1 you actually lose charge preheating, even if plugged in. You lose almost a percent every 2 minutes in very cold temps, 30 minutes of preheating and you could be down 15%, which means you'd be starting your day at 85% instead of 100%, not so great if you need every percent. With ~18a L2 you break even using morning warmup, with 27.5a(or the max a Leaf will take) you can actually full preheat and gain about 10%/hr, very nice indeed. With 27.5a not preheating you gain ~30%/hr, again in ~3hrs you can basically fully charge your car. Much better than the 20+hrs using the OEM L1 EVSE.
Living in a much warmer climate is a different story, there you might be able to get by with just L1 and be quite happy
If I were the OP I'd just go with the outside house outlet, people do it all the time with RVs and don't seem to have an issue. Personally I went with either a L14-30 or L6-30 outlet for my outside outlets but then your limited to ~24a L2, you need a 14-50 outlet(which is a fair amount larger and what most RVs use) to be able to pull 30a continuous.
Note it is possible to find 14-50 extension cords, they aren't cheap($100+ for a 20' model) and they are quite thick, but still cheaper than $2800. Note personally I would not run an extension cord where you mow or over a sidewalk, but thats just me. I guess if you coil it up when not in use it might be OK, thats the OPs call.