This is a very interesting question. There are certainly a few ways in which the current system is undesirable:
(1) As already mentioned, it takes up unnecessary time going between the browsers. In particular, if you finish level A and browse to level B, and then want to view a replay for B, you have to do the A-to-B navigating all over again. This doesn't take much time in itself, but it adds up to a fair amount of annoyance over the entire play of a levelpack (with occasional play of other levels in between).
(2) Uploading levels and replays to dropbox or forum: you have to go
up four levels and back down if your previous upload was a level and you now want to upload a replay.
(3) If your play session is interrupted and you save a replay, you can't resume from the replay and have it count as a success.
A merged browser would certainly address (1), not sure about the others. It depends on exactly how the browser ends up working. Would the current folder structure also be changed to a single folder for levels and replays?
In the old Lix, I liked to have a single folder of favourite replays for re-watching for fun. This is an esoteric use case and doesn't need to be supported.
I don't like the idea of a separate "hack browser" -- everything you've mentioned sounds like it could and should be a function available in the main browser. You certainly
do want to be able to maintain and manage your replays, whether the pack you're playing is your own or someone else's. D Lix currently has space for six large buttons, of which only four are used. The space for each large button could be given to four small ones. "Delete" and "Back" could become small buttons.