Has anyone cycled in East Iceland, and if so what tires would you recommend. I'm planning to ride NR.1 around Iceland so 90% will be on asphalt roads, but there will be
some off road on gravel. I'm going to put some 35mm tyres on my bike, and I'm trying to choose between Schwalbe Marathon Extremes and regular Marathons. Any other 700x35mm tire suggestions are welcome.

I recently bought a pair of Marathon Plus tires, 26x1.5 inch. I ride a combination of paved, loose gravel and hardpacked county gravel roads. So far, so good. But then, in addition to the puncture resistance built into the Marathon Plus, I installed Slime tubes (a little heavy, yes) for safety.

I haven't had to walk home yet.

I went with Marathon Duremes because of the claimed combination of puncture protection, some off road ability and speed.The 700x35 folding tire is 475g, which is quite light for a tough tire with fairly deep tread. In the end I emphasized good performance on wet roads over off road as I'll be on asphalt 90% of the time. The Marathon, Marathon plus etc are a bit heavy and the I wanted a bit more tread, but I was concerned that the Extreme wouldn't be good on the road with it's very aggressive tread.

I'm anything but an expert on road cycling. I don't understand the relationship between having some tread like the Marathon Plus tires, and smooth tread tires like I suppose your Marathon Duremes must be - vs. capability on pavement.

Oh, I like siping on automotive tires, for increasing grip on the road surface, especially on wet or icy roads... https://en.wikipedia.org/wiki/Siping_(rubber))

Has anyone ever siped bicycle tires? If so what were the results?

I always thought that some tread was desirable, for traction on moist surfaces as well as dry.

Ive ridden lots of gravel in Denmark/Norway (full touring load) with 32mm Marathon. The tyre performed well but the size was too narrow. It should work just fine in a 35-38mm size.
I only experienced one flat from a flint shard (not a rock you find in Iceland). I was able to ride up some very steep gravel trails and control the descent (with care).
The Marathon Plus will be more resistance BUT I'm not sure it is necessary and it is a notoriously tight fit to most rims and quite a bit heavier.

Tread on bike tyres is only of use on a loose surface such as gravel roads.

I did some research on the dirt roads in East Iceland and they are well maintained. So as I won't be going too far into the interior and on tarmac 90% of the time I went with 700x35 Marathon Duremes. They look like a good choice as they are quite light (550g) and are claimed to be very tough and roll well on roads while having enough tread to give grip on gravel. When they arrived in the mail I found that the store had send me Extremes, but they immediately sent out some replacement Duremes, but it gave me a good chance to compare the tires.

The Extreme has twice the tread depth of the Duremes ad I wouldn't like to ride them for too long on the road. The Dureme's tread is definitely deeper and chunkier than that on Panaracer Tour Guards and the sidewalls are far tougher. The center of the Dureme looks good for roads and the side tread will work well on dirt.

Continental travel contract is a mixed tread, mostly slick for pavement, row of edge knobs that only contact when surface softens, like gravel.. 'gator skin' sidewall reinforcing, for load carrying ..

It's their International adventure Touring tire.

steel bead , but a kevlar may be offered in a folding version for spare # 3 and perhaps 4.
I have packed stiff tires as spares draping them over the rear load, so that is accommodated.

in 700- 35 ish and 26- 1.75"

Yes I looked at those too, and they look good, with similar specs to the Duremes. I just got back form doing some shopping and the Duremes dealt well with the light covering of snow on the ground.

Given the claimed toughness of the Duremes, my desire to travel lightly and carrying tire boots, I don't bother with a spare tire.