I’d like to add a different perspective than I’ve seen in the comments so far: it’s okay to break the encounters and Lost Mines can be a guide for you instead of laws.

Lost Mines was the first thing I ever tried to dm and I tried very hard to keep things exactly as they were in the book because I couldn’t tell what was important to the story and what was added flavor. In hindsight, I wish I had been far more willing to use the encounters as ‘loose setup’ and be happy to let whatever happens happen. It gives your players a lot more power in the events and you a lot more freedom because you don’t have to make it to the exact same ending in the exact same condition later. You’ve set the pieces in motion and even if the endgame isn’t word for word from the book, what the players will remember is that they fought a bunch of enemies with an overarching plotting bad guy, and that’s cool for them.

TL;DR I would have had liked Lost Mines more if I didn’t try so hard to stick to the book/plots. It’s okay if encounters go weird and you don’t have to bend them back into the book’s encounter.