Heh, I've gotten sunburnt in Western Ireland, so I assure you they do indeed have some sunny days! (It's just that all of them, for the entire year, were apparently during the week I was visiting...seriously, it only rained once while I was there.)
Western coast of Ireland in the midsummer sunlight:
But yes, I would expect the climate to be mitigated in some way, so it's milder than you might expect for the latitude. But it's definitely not so mild as to have the growing season of Florida or southern California or the Mediterranean or anything like that.
We don't have to give the elves of Gondolin greenhouses just so they can feed themselves. But they can certainly be using agricultural tricks to extend the growing season or get a more bounteous crop. Regardless, pruning fruit trees is something you do in any climate - the style of pruning is specific to the type of fruit tree and the particular needs for that plant, but it's part of caring for them to prune them.
Back to Nick's apartment tower - the emphasis on outdoor spaces and balconies and growing things seems good to me. There is also a communal living arrangement suggested here - no kitchens in the individual living quarters, so there must be a central kitchen/dining hall where they gather for meals. So, while most of Gondolin might be more spacious villas, in the areas like this we could expect more space-premium agriculture.
As for the structural integrity of that tower supporting trees growing in it, I will point out that stone can take a lot of load before it's an issue, and the biggest risk would likely be the tree roots getting into the stone blocks of the tower (or the pipes) and disrupting things.