Normally I'd say that this kind of use is a stretch, but in this case I'd say it works, for one very critical reason: you, the DM, are on board with this.
In my own game I would rule against it: a glyph on a surface is measured in feet in the spell's description, and regardless of it being vague otherwise, I would feel quite within my responsibility as the arbiter of the game reality to say that something so complex that it's normally measured in feet could not in any way be reduced down to mere fractions of an inch. (At this point I would also point out to the player that if they're determined, they can just research a new spell that does what they want directly instead of trying to press an ill-suited spell into service.)
However, this is your game, not mine, and the same principle applies: you are responsible for arbiting the game reality. And in this case, you are clearly on board with the player's use of the spell. You even have supplied a cooperative NPC dwarf to get around the problems of engineering that took centuries to solve in our own history of firearms!
The only issue seems to be that you have one (or more?) other player who isn't so on board. In this case, since you are willing to grant the use of the spell, your problem is not the spell, it's mollifying the objecting player(s).
To do that, reassure them that the ruling is in the best interest of the game being fun, which is the ultimate goal of play. "The spell is vague on that point. It's obviously not casting it in an object, so the object rules don't seem to apply, just the surface ones. It's a very small surface, but I'm willing to allow it because it's vague enough that it isn't clearly improper, and because the result will be a lot of fun." Modify as appropriate, based on your judgement and your knowledge of your friends.
If that approach doesn't work, there is probably a fresh question to be asked regarding the social situation surrounding this.
All light sources from magical spells and effects can illuminate a Darkness spell and that includes light from items such as Flame Tongue (RAW).
Of course if light created by magical items can pierce if not dispel magical darkness we would need to calculate their level based on rarity, which to my knowledge is not present.
A good reference however is the Spell scroll conversion on page 200 of the DMG. In it rare scrolls (which are technically considered magic items) harbour spells from 4th and 5th level. Considering Flame tongue is a rare sword, it would be up to your discretion to choose whether to assign it a level of 4 or 5.
That said Jeremy Crawford's statement on twitter implies that only spells cast by characters illuminate magical darkness, which I think makes for more engaging encounters.
Matt Freeman: @JeremyECrawford Does light from a magic weapon’s characteristics (not via a spell) count as magical that can illuminate a Darkness spell?
Jeremy Crawford: Darkness cares only about light created by a spell. #DnD
Best Answer
How cultures function is up to a given DM/Game World/Table
Marriage customs and habits can be expected to vary from place to place in a given game world, if our own world is any indicator. Marriage customs would be expected to vary between Humans, Dwarves, Orcs, Hobgoblins, Giants, Dragons, etc.
That consideration, that marriage is as much "in-game-world-cultural" as it is mechanical (via the ceremony spell), leaves unspecified the norms that are a baseline to reference the effects of the ceremony spell, and the after effects.
If the marriage is polygamous, and only one spouse dies, do all of the other spouses count as being widowed?
Probably, but that would vary by culture. A reasonable ruling would be that to form a new union, they all can marry a new spouse once the courtship/wooing takes place and a marriage ceremony is arranged/scheduled/agreed.
Is there a maximum number of times a creature can qualify as being widowed? Or can they go Henry VIII style if their spouses keep dying and they repeatedly get remarried?
That will vary with culture, so work with the DM on this specific case. As long as the ex-queen stays dead, the spell doesn't seem to differentiate, nor pose any limit. Cultural considerations might, on the other hand, create resistance, friction, or even grounds for a revolt.
If a spouse dies, and later comes back to life, are the other spouses still considered widowed?
While this will again vary with culture, the return of the spouse from the dead represents a great many complications to a marriage.
If they have been remarried by that spell, there's a new bond formed among those who were married, and the returning creature isn't part of it by a literal reading of the spell's text.
In a world where resurrection magic is very common, you would expect to find different rules than in a world with low magic where only the very rich, the very lucky, or the very powerful have access to raise dead or resurrection spells. You could also expect each culture to have developed norms and "statutes of limitations" to govern such occurrences.
Fleshing out your game world with these kinds of details is part of the fun of playing D&D.