1) Yes, technically doors can be broken via other methods - their transform tables allow modification from other sources than alteration. However, I've only implemented modification from alteration, so presently nothing else can actually do this. Broken doors are a flavor for now, then, but in theory, eventually monsters will exist who will be able to break down obstacles between themselves and the player, and the player should have new ways of affecting their environment.
2) Currently the only way to improve your damage is by evaluating enemy damage type strengths and weaknesses and selecting spells which are more effective against your particular target. Casting speed can be boosted, which is useful to some extent, but probably better would be raw damage increases or chances of saving reagents - It should certainly be possible to implement damage boosts with the current system, but reagent modification will be more difficult to do.
Hmm, 6 or 7 fonts in 13 floors might be a bit more common than I intended. I'll probably revise that rarity upward, but not as far as it was before. However, it could just be a very lucky RNG - certainly, a treasure room with only broken chests is extremely rare. Loot containers, when generated, have a small chance of reverting to a different type, and for everything in the room to manage that is rather incredible.
Light going out is an interruption, so it will stop an interruptible action like picking up an item. Currently, any interruption other than of studying a grimoire will only display the interrupt message, but nothing specific to the item or action. In the development version, I've just added new messages to make it more clear to the player that their interrupted action was not completed, so this should be fixed for the next release.
And yes, the masks are a bit poorly spaced on health - I'll look into rebalancing those as well, like I did for Sanity.
Congratulations on winning - I notice you have a couple of Grimoires which you could read well enough to cast some spells from, and quite alot of reagents... were you able to make use of any of them? I see you finished the game with the starting Grimoire, and I'm hoping that's only because of the overabundance of mercury you found on the run.