In
this post, CDM says:
Limitations on items are placed when you obtaining them (pulling, mall, taking out of DC), and not when you use them. This means, that any future monthly leaderboard will simply count runs starting on the 1st or later, so that you don't have access to items that expired that month. This was just worlds simpler than checking your equipment at rollover, and every place you can use, equip, etc an item.
That says, to me, that if you manage to have an item in your inventory on a Trendy run, there are no limitations on using, equipping, crafting, drinking, what have you; the item is assumed to have been fashionable when you put it in inventory, and that is good enough. In that sense, your Terrarium is just a source of items that you can pull, and you are prevented from taking un-fashionable familiars out.
I'd suggest that we filter un-trendy familiars in the Gear Changer - as now - and also filter any un-trendy item from the storage list, so you can't even see them.
We need to detect failed attempts to pull un-trendy items from storage, store, display, and terrarium and not change our internal state.
We do NOT need to prevent people from using or eating or drinking un-trendy items since, according to CDM, KoL does not prevent that; if you have the item in inventory, it was fashionable when you got it, and using it will not violate your path - although you had to have crossed a month boundary for that to be possible, and you do not qualify for a leaderboard, if you do that.