Ha ha, look at those html errors in my post. Not sure what it's complaining about.
Just thought of a potential problem. If two sessions mark different history items with the same shortcut letter, simply merging the entries will result in duplicate marks.
I guess in that case, the later mark could win. No entries would be lost, but one wouldn't have the mark that was set.
I just tried adding a duplicate mark for my filter history by editing ztw.hst. When I started ztree, I can see them both in the list with the same mark letter, but when I try to use it, it ignores the one higher in the list. (Curiously, the list is in the reverse order of the file.)
So simple merging works, but could cause confusion.