A with changes possibility might be to change the Entry Creation Page a little instead. What about adding a check box to disable the date setter, and adding a check box to enable a new module that gave the option of manually numbering the entries.

It's already possible to change the dates on entries, so the numbering module might not work very differently. But the maximum number of allowed journal entries in one journal would have to be determined, because we'd have to number backward, to keep the largest number on top in the journal. (That's how I see the latest/ newest date - a larger number.)

The worst problem I've read so far is the mention of the limitations of the cache of the posts/journals and the servers. If I understand the problem correctly, most journals people only go to the top page or two to catch up. This journal people would consistently be going to various sections and almost ALWAYS going deeply past the first few pages. Although...for this particular journal that will still be the case after a while, it will just be people going to the most recent entries on top and going all the way to the back and reading forward. So idk...?