I'm new to Dead Cells (I've only had about 20 hours) and I have some questions:

• When I am in the space between levels where I can pay with cells. When I get weapons from the collector using cells, are those weapons forever? In that case, can I get that weapon again from the collector in another run so I can use it in that run, or can I only use it when I get it randomly at the start of a game?
• Right after using the collector, when I go to obtain a mutation, most of the time it only gives me a choice between three red mutations, the others appear blocked in most levels. Why does this happen? How do I unlock it? Why are they blocked in some levels and not in others?
• Sometimes, in some levels, there are secret areas where the entrance is very small, where normally one would just roll in, but said entrance is not at ground level, and no matter how much I jump to try to grab myself, I can't do it. Is there Is there any way to get in there or is it just a way out? It's like there's some kind of ability that lets you grab onto ledges or something.
• I managed to beat the final boss of the Castlevania DLC (I won't give more details to avoid spoilers) I told a friend and he told me that the DLC must be easier than the main game. Is this true? Is it easier to beat the final boss of the Castlevania DLC than the main one?

Thank you.