If you fermented in a sealed container without any pressure release, the thing would blow up. So I'd say unsealed is a requirement.
I have two fermenters, both airlock style ones. My main one had the grommet perish ages ago which I noted above. The airlock never bubbled on it anyway, but since the grommet perished I just put tape over the hole and back the lid off slightly. Gas escapes, nothing gets in. It's opaque but the lid is see through so I can still observe foam/condensation forming. I can also see the foam through the side.
Point being, airlocks aren't necessary and they shouldn't be relied on as a sign of activity because 1, any leak in a seal stops them bubbling, and 2, they can bubble for reasons other than fermentation. All they really indicate is gas passing through, and while a lot of times it will be from fermentation, it's not always. Ignore it, look for visible signs and use your hydrometer.
My standard ones are about 5-6%, but mashed higher around 67. I don't think I'd ever make a beer with 20% crystal.
I'm also led to believe that when crystal is mashed, the enzymes in the base malt reduce its influence compared to steeping it on its own where no enzymes are present. I haven't bothered testing this theory as I'm happy with the current outcomes.
Just following the recipe in the charcuterie book mate, not sure what the boiling is for but anyway. Although I obviously leave it in there longer than two days like it says. Maybe their loins are different but it never fully penetrates in two days.
I'm not sure where you are getting these recipes from either. Most IPAs are low on crystal malts, 2 to 5% or so. Otherwise you don't get the dryness that you mention. SNPA is about 10%, and there are beers with more than 10%, but 20% is not common at all, from what I've seen of searching recipes over the last ten years.