Does this sound a bit like "bolai" to anyone else?
Yes it sounds like bolai to me too.
Mulai= start, begin.
??= finish, end
"Mulai" >< "Selesai".
Can it mean "let's start"?
Yes, it can. More accurate translation of "let's start" is "ayo mulai"
Can just the word Mulai be used as an imperative?
"Started" should be accepted