The wheels I got with 4x100 came from Germany and I'm in the US, so I didn't think I could exercise any warranty. Makes sense though. Maybe the shop that drills them can give you some kind of guarrantee if that is important to you. I'm never concerned about warrantees with Alfa related mods; it's always at my own risk.
My conversion actually didn't use inserts. The guy just drilled out a bigger hole centered 1mm inward and re did the cone seat. He said for larger conversions he needs to use inserts because he has to drill a bigger hole. I used a lug nut that had a wider lip that extended out a little since the original nuts sat inside the new cone a little. I'd prefer the inserts since they are identical in geometry to a wheel hole. I may go back to him at the next tire change. He charged me $150US for all 4 wheels.
Concerning the spare, it is still in the 4x98 wheel pattern, so there is no problem.
Some people have suggested removing the hubs from the car and having a shop put the wheel studs on at a new location of 100mm or another popular PCD. They could stagger the new studs on some new holes at 45 deg off of the originals. I thought this to be a lot more work and I'm only chosing the wheel I like once. If you are swapping wheels around often, this could be the solution. Otherwise, just pick the wheels you want your car to have, buy them and have them drilled. I have heard racers doing this and had no complaints over strength.
There is not a lot available in 4x98. I think Ronal will do it on some models. Or all the regular Alfa wheels - Panasports, Cromadoras, etc. You may find some more Fiat or Lancia stuff if you are in Europe. Maybe not in 17" though. I don't know what some of the later Fiats and Lancias had of interest, since we didn't get the later stuff.