I have just tried playing single player with all mods unsubscribed.
I have sprint bound to the -left Ctrl- key,
Crouch bound to the -C- key,
Toggle walk bound to the -backspace- key
Block/offhand- bound to -left shift- key
Heavy attack bound to -RMB-
Heavy attacks wind up but get stuck and have to be unequipped.
When I try to rebind the mount sprint to -left Ctrl- I get a message that there is a conflict and it deletes my sprint input on my regular movement keys after giving a message about a conflict instead of keeping both keybindings like it does when there is a conflict with build mode keys.
The walk toggle function works with both movement and mounted mode, this function has no issues.