1: I think it's fine to scout around super far in my opinion. It doesn't add too much power into the game because you can usually see people on the horizon a good distance off just on yourself. Unless the imp also triggers ambushes, then there might be a problem.
That is of course a matter of taste. I would also favor to go in line with tabletop rules (100 ft leash), mostly otherwise it removes the need for PC scouts (rogue, ranger, etc.)
2: A warlock's imp is able to attack normally in D&D 5e so that's no true shock. Enemies not attacking it back is a really weird though. That sounds more like a bug, or enemies are designed to just flat out ignore familiars and go for the summoners.
Warlock's Pact of the Chain familiars are able to attack, but at the cost of one attack of the warlock. PHB, p 107: "Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to use its reaction to make one attack of its own."
3: Making independent skill checks actually would be awesome for the warlock familiar and not the other classes familiars spells.
Normally that is not needed, as Pact of the Chain forms are just overall better at skills than normal animal familiar (except maybe owl and hawk, but they are not in the game)
4: I'm actually surprised they don't have darkvision already on imps. They normally should have darkvision as powerful as a drow.