You must use only "one". The article "a" functions (weakly) as a numeral also, so you will have an unpleasant redundancy in the sentence.
I heard that coats were invented for this reason and therefore wearing one will surely keep you warm.
"One" substitutes "a coat", not "coat". This is an additional reason why "a" is not needed.
"A one" is not usable ...
It means exactly what it says. If there are one or more people who have come from somewhere else, then it describes the first man to have arrived.
It doesn't necessarily mean that he has just arrived.
If the event were being commentated on, the sentence could be spoken when the man appears. Many hours later, especially if no other man has arrived, the ...
I would assume them refers to projects, because to-dos and tasks are pretty much synonymous. But the language here is somewhat confusing and ambiguous, as you can tell. Maybe it would be better understood if written this way:
In the app, you can easily group to-dos into projects and drag them within and between projects.