In editor you can easily choose between mineable stonemountains and ones that are not mineable. They look very similar. With good placement and shaping it could look natural and you can be sure that sonemasons will mine only that place and you can predict more or less how much time it would take to make a gap. (I agree, that there's too much stone in that picture).
If you mixed both types of stone it could be misleading for players who might try to rush iron and build most of their quarries next to iron (which would run out pretty soon).
About slowing down economy in 60 PT, I'm not so sure about it. Of course, going for extra iron montains would mean, that it slows down your corn production, but maybe with trading it wouldn't be that bad. Additional storehouse wouldn't be needed if you trade extra stone.
In order to have decent army at PT you must play extra greedy, timing your buildings pretty well (building them when they are needed), going out of your way to have that extra iron
is going to slow you down. For comparison look at any map where iron is quite far from your starting point - you just have make some compromise, you can't have both.
*I made a small test. With 3 quarries, you mine 300 stone in 30 minutes. When trading it, you'll get 60 corn. For comparison, 3 farms would give you about 70 corn after 30 minutes. Difference isn't that big, of course it's better to have farms, but when going for extra iron, having extra corn at the same time is pretty useful I think. But still, it's only a test.
You have to build a farm anyway to unlock other buildings, your stone will eventually run out, you have to train lots of serfs (lets assume one farm needs one serf, one corn needs five stones), deal with possible traffic jams, not to mention additional iron mines won't just spit out swords and shields and so on, you need coal mines, smelters, etc.
Every strategy that relies heavily on market is essentially an all-in strategy, you win fast or die slowly because of starvation and lack of real economy.
I just don't see it working in multiplayer and more importantly you can achieve the same effect by just pacing (additional) iron mines a bit farther than normal.