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Ah yeah, those bikes are definitely designed for drilled guards, and they look so much neater too. No bright silver clamp standing out against black mudguard.
Fwiw, that hole is a little vulnerable to cracking/damage from rattling so centre punch, then sharp drill, loctite on the bolt and then you can get a reinforcing plate to go inside the mudguard, works a treat. Sorry to teach you to suck eggs!
Looks neato! So:
1) if you have SKS guards they supply some serrated plastic washers that are great for tricky angles as they have some flex. I imagine SJS cycles will have some, if not holler and I’ll post you a few.
2) just like Velo Orange/Honjo/Berthoud I think you should be fine to drill through the guard and bolt through with a spacer in between. I think @pish has done something similar.
3) maybe with an M5 bolt for future-proofing/accommodating a dynamo light attachment?