From the rules for 5e
Familiar. The imp can enter into a contract to serve another creature as a familiar, forming a telepathic bond with its willing master. While the two are bonded, the master can sense what the imp senses as long as they are within 1 mile of each other. While the imp is within 10 feet of its master, the master shares the imp’s Magic Resistance trait. If its master violates the terms of the contract, the imp can end its service as a familiar, ending the telepathic bond.
so I don't know what the scale of stuff is in BG 3 but a mile is plenty. Imps are powerful scouts
for combat, you are right. "Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one attack with its reaction." so it should be the Warlock giving up an attack to have the Imp attack. What is missing is the ability for a familiar to do the help action to give someone advantage to attack which would still be a huge benefit in combat.