I think 10-12mm spacers is probably all you need, as I’m starting to recall when I was considering different wheel sizes, that most of the 7” wheels getting put on Spiders had offsets of 20mm. So if you have 35mm offset, the closer you can get that to 20mm (with spacers) the closer you are getting to something that will work.
You are saying that, in theory, 15mm spacers are ideal as they result in an offset of 20mm, but there is a chance that 10mm spacers and a resulting offset of 25mm will be enough not to rub on the inside well. If I just took off the spacers, 35 will most certainly be rubbing on the inside. Correct? In that case, I will take the risk and give it a try with 10mm spacers now, hoping that the 12mm I'll be losing on the outside will be enough to clear the tire from the fender while not being too much to make it contact the inside.
But I should add there is something more to this, and that is wheel stud length. As you space the wheels out ore, you increasingly lose wheel stud length.
So you as you space the wheels out, you need to make sure you respect minimum thread engagement requires, which I think is roughly 7-8 full turns.
I'll just buy the studs in a kit with the wheel spacers to avoid that issue alltogether. I don't want to try with washers, first of all I would need to buy those first and secondly, I don't want to drive the car with washers as an interim solution. Turning the wheel while jacked up isn't good enough to test because I can't account for bumps that way.
Can you confirm that you have adapters and not spacers. To be clear, spacers are what you see in the link you provided. Adapters are much thicker and come with 4 through holes and 4 studs. You bolt the spacers to your hub and then your wheel to the spacer.
You are right, I have adapters. Does that change anything?
I just spoke to Alfaholics and they suggested "The offset we recommend for the Spider on 7x15 is ET29 so for your ET35 wheels a 5mm spacer will be about right." They don't expect any rubbing without the spacers, either. I'll follow their recommendations and will update you on the outcome!