Well, it may be too nit-picky, but the "glassmaking" jumps out as unrealistic, but admittedly only to anyone who cares about medieval glass-making. Venice had the best glass, followed by Arabia and then Russia, with anything else being 3rd rate if it even existed (which was rare). Both potash and fine sand are necessary for glass and kept an aggressive lock on it, and the best quality potash comes from certain trees - and "trees" are not an icelandic forte. But "trees" are not enough - England and France had plenty, and they didn't make glass. If you did allow it, it should(?) be very low quality indeed, both from lack of proper raw material (the right kind of trees) and lack of skill, skill that Venice would go to war to maintain its monopoly over.
And "shipbuilding" requires, again, wood - that's still not Iceland.
Fish is good for any seaside-ish area. Even "inland", an Icelandic covenant could have a controlling interest in some fishing fleet, maybe using magic (overtly or covertly) to increase the profit, either by increasing the haul, lowering the risk/costs, or whatever.
Likewise, salt - seawater and heat, and you got it aplenty. Magical heat is easy, and only increases profit. (And it's far more valuable than it is today - but if they try to start up their own saltworks, they may be stepping on some very affluent toes.)
In City & Guild it lists, for the goods of Iceland*... falcons. Which is news to me, but who am I to gainsay it?
(* Under the geographical heading of "Places So Far as to Verge on the Mythical"...)
That in the line above in that same section it also lists "gemstones" as coming from "everywhere" (... anywhere so far as to verge on the mythical, that is). For Iceland, "gems" are scarce in reality, but you could go with spar crystal (aka calcite, a light refracting), carnelian, feldspar (including sunstone), and, of course, obsidian. And you could always pretend that diamonds or other high-value gems are found in the volcanoes there - they aren't, but it works for fiction.
And/or pumice stone - that's lightweight, and I'd guess (don't know for sure) that Iceland has some of that.
Each gives a slightly different feel to the Covenant, how they acquire the product, who they sell to, who does the work for them, etc.
Other than that, for natural resources, you're screwed. Iceland is aptly named - but that doesn't mean that (especially with a little magic) the magic couldn't lean toward some industry, some "processed goods" that they could then sell across the island or export back to Europe. They could go with anything that the locals produce, only better - parchment from sheep, tallow or leather from cattle, Or with a good smith, ingots of raw metal (iron, brass or bronze) from Europe and a forge (perhaps a magical one? easy enough) they could produce metal products. (If they need to import the raw materials, they're selling that locally, and aren't selling those products back to Europe - their competition there doesn't have to bear the same transportation costs to and from.)
For that matter, their "income" doesn't have to be local - it could be anything on the nearest mainland with more plentiful resources - trees for lumber or charcoal, furs or leather, wool, salt, foodstuffs, even something simple like rope or cloth - and shipping the product to them where they then distribute it.
Or (if you're comfortable with their access to ships) it could be a small shipping company itself - this would keep them more connected with the mainland (which might be good or bad, depending on your image of the future saga). That would feel better than the shipbuilding (unless the shipbuilders were in Europe - again, anything "there" is far easier to believe, if not more practical to oversee).
If they could build a magical greenhouse (not that hard, but some work, admittedly), they could go with herbs, and/or bees, and the honey and wax would both be a luxury item across that land. Once magic is brought into the mix, many of these obstacles are irrelevant.
If the Healer Bonisagus wanted, they could create a "house of healing" - especially if there were thermal vents that might be found to have magical healing properties, but could also be used as a "cover" for Hermetic effects. Hot pools, saunas - now we're talkin'.
Any House interested in exploration and new lands (altho' they'd send a representative of that House, I'd think - PC or NPC), or the nearest existing Tribunal as a whole (hoping to "annex" Iceland to their own Tribunal in the future.
Possibly a collection of their own Parens and some other magi for their own personal reasons, as investment, political maneuver, or as a friendly base for their own purposes. Nothing stops private ventures in such matters.
House sponsors:
o Bjornaer: Wild is good, tho' a bit thin on animals
o Bonisagus: would have to be feeling very philanthropic, but if it expands the Order, sure.
o Criamon: who is to say "no"?
o Ex Misc: as above
o Flambeau: Offensively or Defensively against the Order of Odin, or as punishment for a specific mage
o Guernicus - I'd have to say "no" - cost/benefit just doesn't support it imo.
o Jerbiton: Likewise, a tough sell - just a bit sparse on culture and population centers
o Mercere: Exploration is good
o Merenita: Wild = fae, sure
o Tremere: "Ha ha, now we have an unstoppable majority of votes in all of Iceland!..." (???) sure, I guess...
o Tytalus: Challenge is good
o Verditius: "Maybe NOW no one will bother me..." er, but as a House... maybe they thought some insight to forging could be gained from the volcanoes?
Sure, of course, and yes.
They could be indebted to anyone mundane or Hermetic, in Iceland or back in Europe proper, either as "rent" for existing in Iceland or in repayment of set-up costs.
More mythic elements are always good. I'd lean toward odd, fire and/or ice-related Fae and old-school giants and dwarves.
They need specialists, anyone to upkeep what they have and don't want to constantly replace, and anyone to help build/expand/maintain the covenant grounds - smith, carpenter, mason, a master miner if they're going into the volcano, clothier, armorer, cartwright, chandler, shoemaker, cook, mundane nurse/healer (unless anyone with a cut finger or a cough will receive vis-based healing), butcher, parchment maker, etc etc etc. Stablehands/teamsters/muleskinners if they have mules/horses. This list can be verrrry long.
Plus a castellan to run the grounds, a chamberlain to head the servants, the steward to oversee the covenant finances, and of course grogs and a grog sergeant.
Plus all the mundane gear, food and raw materials to get them thru the first year that they can't make themselves.
I, myself, would also toss in a (semi) portable enchantment or two to get them started and survive the first winter - maybe a brazier or chandelier for their main hall, or a long-range communication toy to trade messages with the sponsoring party. And include in the library some of the more obvious spells - light, heat, clearing snow and ice, etc. If you're not sure, let them negotiate that In Character - more goodies for more debt, as much as they want.
If you want to have the story revolve around creating the covenant, this is a long row to hoe; if you want to get to other story arcs, then they need what they need, and should either be handed it (with an appropriate debt, natch), or be able to acquire it somehow in the first year(s) of the saga.
Are you a chapter house?
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