Yes, I'm using MLO for Windows.
1. I know about Alt-Enter after typing, if I want to parse dates, but why not parse them by default by just pressing Enter (how it is done in Todoist, for example)? Of course, sometimes we may want the text no to be parsed, but I think it happens not as often, and Alt + Enter would be more appropriate in this case. Or make an option to choose a custom hotkey. (Just my 2 cents).
2. To me "Enter" seems like the most obvious choice for Next task (New line). It's like this in MS Word or any other word processor, Taskade, Dynalist, etc. Same is with "Tab" to make an indent. It actually goes way back to typewriters. I don't know what percentage of the world's population used and uses those keys when making bulleted on numbered lists in word processors. Even MLO uses Enter for next task in Rapid Task Entry window (that's what makes the entry rapid!). And Space to make a subtask, which is not as customary as Tab, but still makes it much faster.
3. Which key combos would you suggest for "New task", Indent > "New subtask", Outdent > "Make it a parent task again"? Unfortunately, on my keyboard "Ins" is sharing the same key as "Del" and should work when "Function" key is pressed, but for some reason it does not.