I am a new user but see there is an option to sort a branch under the context menu 'advanced - sort subtasks'. I guess this was implemented since OP's ancient question!
I confess that MLO is very powerful/configurable, and hence complex - I am still learning! So all I can report is my method:
I prefer the outliner to sort top level parents according to importance etc, while the tasks (children aka leaves) within each parent are manually set up in a sequence (no need for priority fields etc). MLO knows to present the first/next incomplete one in the 'todo' list.
But if a complex project has multiple parallel tasks I can see that sorting them per OP's question is needed (or maybe a filter?) to sort the wood from the trees. Warning - I have not tested this and see the menu says 'this cannot be undone' - I would be afraid of borking the entire tree by mistake. "With great power... etc".
My bolt-on question (perhaps deserves a new thread...) is that I notice that however I tweak the settings, the todo list always bunches dated items right at the top (including recurring tasks), and non-dated below, very rarely putting non-dated items among the dated ones.
This is awkward for me bc I want to fulfill the opening paragraph of the MLO introduction - i.e. to prioritise high value projects among the regular fluff of life and work.
I feel that setting a 'date' imposes a heavy implication of 'must', whereas this is not true at all. E.g. if I set a date to remind me to do a recurring task, it might be mundane like 'take clothes for pressing'. The settings for priority etc don't seem to have enough influence over the due date.
Does anyone else feel this way?