SPOILER AND A LOT OF SPECULATION TOO! I think the reason why in the first spiral or timeline where the nuke explodes Entrati did it in 1999 since radiation is one of the weaknesses of the Murmurs and nukes are just that, Entrati knew the man in the wall was going to chase after him and so he goes there, the Kalymos sequence is like a second plan in case what he does in 1999 fails or can't complete, and the reason the drifter is the one to go is because the deal with wally doesn't work with him since as per eternalism the drifter is the operator who should've died but didn't since he got trapped in between the void, basically the drifter is dead (when wally did the deal with the operator and killed all timelines of him/her to give the void powers) and alive at the same time (since he/she was trapped in the void wally powers couldn't reach him/her or my guess is that because the drifter was always dying and going back in duviri and all of it was in the void, it created a state for the drifter to not die as we know the void can create something like duviri out of just thinking about it) that's why he can use his paradox to revert time or I guess to go to another timeline.
SPOILER AND A LOT OF SPECULATION TOO!
I think the reason why in the first spiral or timeline where the nuke explodes Entrati did it in 1999 since radiation is one of the weaknesses of the Murmurs and nukes are just that, Entrati knew the man in the wall was going to chase after him and so he goes there, the Kalymos sequence is like a second plan in case what he does in 1999 fails or can't complete, and the reason the drifter is the one to go is because the deal with wally doesn't work with him since as per eternalism the drifter is the operator who should've died but didn't since he got trapped in between the void, basically the drifter is dead (when wally did the deal with the operator and killed all timelines of him/her to give the void powers) and alive at the same time (since he/she was trapped in the void wally powers couldn't reach him/her or my guess is that because the drifter was always dying and going back in duviri and all of it was in the void, it created a state for the drifter to not die as we know the void can create something like duviri out of just thinking about it) that's why he can use his paradox to revert time or I guess to go to another timeline.
That is VERY close to what I think as well.