1) not so much that the average human can tell the difference
2) 1M is 1M, but the Helix has variable settings starting at 10k, which makes a difference with some effects like fuzz
3) The apollo has better latency when used with VSTs. It's on-board amps are said to be quite good.
4) Depends what you're doing
I have a Helix Floor which I plug the guitar into. Same zero latency monitoring as the Apollo.
I use a Scarlett 18i20 for an interface as it has many more Ins/Outs and better latency than Helix when used with VSTs.
The Helix is a better guitar device, and can be used live - do you REALLY want to bring your computer to Joe's sleazy Dive Bar'n'Grill?
Keep the Apollo and use both for what they're best at.