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."
That's not the same thing as the Imp having an attack. That is an additional attack action the Warlock's Familiar is able to make on the Warlock's turn. It isn't using an attack action it's using a reaction, while you use an attack action.
Familiars have their own turns, and while most familiars are unable to make attacks, Pact of the Chain special familiars can. So there is nothing wrong with the Imp being able to attack.
Honestly, most of the complaints are just how Pact of the Chain works in Table Top. Imps are pretty broken, and always have been.
The thing that's missing is the ability to cast touch spells through your familiar. In the case of an Imp it doesn't break invisibility because the Imp isn't casting the spell, just channeling it. That is where the 100ft limitation comes in.
