Tyre Rub - Do I need to change for smaller tyres?

Do you mean HGV tyres? They run at 90-130 ish depending whether they're on the truck or the trailer. If you've ever been sitting in your car when an HGV has a blow-out nearby, you'll need clean underwear  .

So, you need to find another tubeless tyre manufacturer that makes a 35mm tyre true to size (if not a little under). Panaracer, Clement, Hutchinson, Bontrager, Specialized all come to mind. Keep the G-One as a replacement for when the rear wears.

You need to be going some to break traction on a Schwalbe 28mm tyre. You could alter the grip by lowering the pressure a bit.

I didn't realise that there was any resale value in used bike tyres.

Why not replace the front tyre as suggested and keep it for when the rear one wears out? 

Try to see if it is the fork or the caliper causing the rub. I put some 28s on my Trek for Roubaix last year which rubbed the same way you're describing. However, changing my 6800 Ultegra calipers for 6700 Ultegras gave me just enough extra room. The 6800 was narrower due to a more 'bulbous' design, for want of a better word.

Lower pressure will help but this is a temporary solution and you need a new (smaller) tyre. Remove current one while its still saleable on ebay as "hardly used".

I had a similar issue on my old TCR, put 25mm schwables on to wide rims and it left very little clearance.. the front was okay, but the rear clearance with the brake caliper was a problem, In the wet I'd pick up debris, which caused rub and scratches, once I had to dig the mud out with a stick it was that clogged. Then at the top of Alp d'huez, the day prior to the marmotte, my rear tyre 'ripped' I'd gone over a very slightly sticky bit of tarmac, picked up a stone which jammed under my caliper and ripped a hole in the tyre casing. 

That 1mm vertical clearance is a tad small in my opinion - pick up some mud or similar and it will drag through and (repetatively) over time wear away some of the crown. The buzzing is no doubt the side to side movement of the wheel - you must have a very small lateral gap too. Similarly, this is not good for the fork in the long run. You have two choices - always remain seated when cycling or fit a narrower tyre. You can possibly still put a 35mm tyre from another manufacturer which actually measures 35mm. Schwalbe are notorious for coming out wider than stated (my 25mm Pro-One's come out at 27.3mm on Ultegra rims). Running a lower tyre pressure will also help a little.

Might be worth sticking a 28mm on the front? No rub (and it's a bit more aero, too ).

With only 1mm of clearence any debris the tyre picks up is going to cause scratches or worse to the forks arch

Have seen this on bikes before, one even had shavings of alloy taken out the bottom of the caliper due to smallish stones stuck to the tyre being dragged through the top of the fork