1 - Yes, you'll hear yourself immediately playing alongside the other players. No, it won't be what they're playing at the same time as you - but it will be in time, so long as you can play to a metronome, and it will fit musically, so long as your phrase length can be accommodated.
2 - They'll hear it later, it will be in time, so long as you can play to a metronome, and it will fit musically, so long as your phrase length can be accommodated.
3 - You hear what you play ONCE, IMMEDIATELY you play it. If something else is happening for you, your set up is faulty.
NINJAM isn't intended as a rehearsal room. You can jam and it's not repetitive (if you know what you're doing with your phrasing and you listen as well as lead).
A random jam.
If you want to rehearse online, that is possible - so long as you don't mind standing around 10 to 15 metres apart whilst you play. Think of it as social distancing. It requires that you have a Jamulus server close enough to you that your overall delay is under 40ms. Then you can connect Reaper to Jamulus client and jam together, in real time. London to Netherlands or Germany is possible, for example, quite easily within the 40ms. For example:
20200525-1805-edit01-pljones-A_T__Jones-nils
I'm in London, UK; A_T__Jones was in the Netherlands; nils was in Germany