Would spacers allow wider rear wheels to fit on the front?

I would like our Giga BEV to have a square 19" wheel/tire setup like that of a Mega BEV but with 429 (Giga) wheels so that I could perform proper tire rotations. For BEV models, BMW offers a square 5" x 19" 427 (Mega) wheel with 155 mm tire setup, so handling and rear tire load must be sufficient. I could just buy 2 5" x 19" 429 wheels and 155/70R19 tires to replace our 5.5" x 19" rear wheels with 175/60R19 tires, but trading my rear wheels for another Giga owner's front wheels would be much less expensive. This would allow the other Giga owner to have a square wheel/tire setup but with wider wheels and tires.

However, I've read that 5.5" rear wheels won't fit on the front due to some sort of physical interference. Would adding spacers to the front allow 5.5" rear wheels with 175 mm tires to fit without interference and to not protrude beyond the edge of the fender?

Place a straightedge across the fender in front and then see how much space 0.5" would move the tire/wheel. It might hit things on bumps or when on full lock. If you put the car up on blocks, you could move a rear tire to the front and at least get a feeling for the clearances. Based on what has been said, though, probably won't work. I'd have to take a closer look, but I think the outside edge stays in the same place, and the wider rear wheel/tire projects more towards the inside. Moving it out is as much or more of an issue as projecting too far in. Unlike a metal fender, you can't roll the edge to get a bit more clearance that direction!

As to buying a new set of 5" rims and tires, will work fine as all of the winter tire sets are square, regardless of the specific i3 model. It might be the tread compound and pattern, but I find the winter tires a bit less dialed in as to their feel on a crowned road or during strong sidewinds. It's subtle, not dramatic, though.

Yes.

A member/owner on the FB group did it, it was very expensive as he lives in Australia.