Hello and welcome.
The stormfist castle guards will always block your entry into the dungeon at this entrance, even if you kill them, I think the dialogue still pops up to prevent you. Plot-wise, it's not important since you can find other ways in (through the sewers).
Aside from
teleporting via hex editor, I can't think of any way to get to these hard to reach places. IIRC Penumbra has her way with you and leaves you standing outside the locked door to the hallway. Maybe use Izakaron's save editor to give yourself the translocate skill, though this may not work pre-wastelands.
The house in the Dark Forest can be reached by tossing teleporter stones, but I don't think there's anything of value in it.