yes alfa's (unless very early) are 4 and 5 stud x 98mm pcd
yes subarus are mostly 5 x 100mm pcd, and yes 2mmm wobble bolts can be used,
some (JDM) japanese fit wheel are made for nuts only, or tuner bolts, not large head bolts like ours, so you could do with checking that first, the nut holes can be very small, so you struggle getting a 19mm socket in!
not a huge issue, and if you really like and want the wheels, go for it, all the nuts studs collars etc are out there and available no problem.
Ive just done similar with my 147 GTA to fit aftermarket wheels, and now have a full stud conversion and tuner nuts, which is loads better IMO
don't forget that the et offset being a higher number, will make the wheel sit tighter in the arch, not stick out more, so watch for inner clearance, or compensate appropriately,
the difference is only 7mm from et 35 to et42 so its not much, but some cars run quite tight on the inside already, and tyre choice can make just as much difference too!
shrew (Wheel Hore!)