The Flask is pretty meh as powerful magic artifacts go. Like, yeah, battle magic in a can, but bound spells are a thing - we're not talking about something uniquely powerful, just a moderately good tactical-level asset. They make them. Nations aren't going to risk war over them. Powerful rulers aren't gonna murder each other over them.
... besides if anyone
was willing to kill us for our magical artifacts, the Flask is... probably the last on the list? Like it's probably our weakest and least significant magic item. I could believe someone being willing to murder us for Branalhune - still stupid, but it's at least worth the effort, not so much the flask - at least not anyone with an actual chance of pulling it off.
That said, we're supposed to not micromanage items, and that probably includes giving them away. And of course even if it's just an item, it's fire magic being offered to the Tsarevitch, which is probably not gonna go over well, and also he probably already has magic items... (his TT Tzar version certainly does)
Dragon Flask can't just clear a room, it can blow apart fortifications. Its not quite "level a country" but its still a very powerful magical artifact. You could terrorize a province with less if you managed to slap together enough brigands.
You know what else can do that? Cannons. The Boyar very likely has access to cannons. And an army. The Tsarevitch very definitely does.
I don't get where we're getting 'enough brigands' from, but you're right, it'd be a significant step up from the kinds of resources brigands have. Because brigands usually don't have many resources. Or numbers, for that matter. The thing is, we're not talking about random brigands. We're talking about a prince with access to far greater resources.
Where'd you get that from? IIRC the one time Mathilde tested it, it destroyed a small clearing. Not a fortress.
It's likely hot and forceful enough to blast down a decent masonry wall. But like I said. Cannons. Or battle magic.
It might be able to create a very, very small breach in a castle wall if it was like a couple inches thick. I genuinely don't know how thick castle walls are.
Meters. That said I think you're underestimating it a bit, and not every fortification is a proper castle. But like, yeah, it's clearly not exactly toppling fortresses.
[X] Investigate the missing leyline
[X] Find the Boyar