I'll address your questions in order, there's a lot of info here.
First, the wheels are 15x7 ET25. The offset of a wheel is measured from the centre of the barrel to the hub, meaning that when the wheel has a positive offset (i.e. any wheel without ungodly dish), making it wider via larger lips makes the offset lower by exactly half of the lip added. In the examples I was giving above, a 7" wide ET25 wheel becomes 7.5" ET19, 8" ET13, 8.5" ET6, 9" ET0, and so on. The offset has to plunge downwards, it's just physics.
Now, why that is bad for you. The hub to fender geometry on an E30 does not allow such aggressive (sub-ET20) offsets, as they will make the wheel protrude from the wheelwell. As we all know from "Euro" styled cars, you can still drive a car like this with narrow tires, as thr tire will be pulled into the fender due to lack of width. In essence, the widest you can make a wheel that started out as 15x7 ET25 fit under an E30s arches with 225/50/15 tires is approximately 15x7.5" ET19. Anything wider than 7.5" and you will be contacting the fender. Of course fenders can always be "encouraged" to accept more tire, but this is up to you on how much you want to spend.
A 16x9 ET0 wheel toned down to 8" of width becomes ET13. In fact, the geometry of that wheel indicates that is has exactly the same hub geometry as a 7" wide ET25 wheel, the numbers I stated above apply directly to this wheel, also. To be clear, what I mean here is that if this wheel had the same 1" lip as the stock 15x7" ET25 wheel, it would also be ET25. It is just a severely widened version with the same dimensions.
Lastly, 225's would have no issue with 8" of wheel. It's just that with the wheels you are considering, if you make them 8" wide they will stick outside the car, and getting properly sized tires on them means you will have to do something about the fenders (pulling, likely painting afterwards, etc). Wheels such as 15x8" ET22 Kosei K1s will take 225's with no problem, since they would be about 1cm further into the car on each corner.
Hope that helped, don't be afraid to ask for any clarification on this.