Could someone explain Ekko to me?

So since seeing the little thing on how his abilities work i am still super confused on his ultimate. It says it makes you untargetable and invulnerable for a bit of time, then you pop back in time get a heal (based of life lost during the ults time frame) and do a big AOE nuke to enemy champions near by you. My question is, can you do anything while the ult i active, like attack use abilities ect.... How the hell does the heal work if you can't be targeted and you can't take damage during the ult?

Also other then a way to save his own ass, how is the ultimate useful for a team? It looks like you get an after image and that's what you port back to when the ult runs out of time. That's cool, but how is that useful? Do you play Ekko as a jungle, pop the ult while running towards the enemies and then when they try to flee, you pop back and AOE them?

Second question, is based around his slow/stun ( his W ability). Does this have a set time frame it will always go off after? like use ability and it will always go off after 2 seconds? or is it based off how far away the alternate Ekko is from where the ability will go off? Also can we talk about it's range? Looks to be an area of effect around the same size as Singed's glue, but at a cast range of like 1200-1500.