I believe I have found the answer to finish this:  

If you read the Oni's stat block it says that invisibility is an innate spell cast and that attacking ends the spell immediately, but in the Imp stat block it says it can be simply invisible: not the invisibility spell but a natural ability with concentration.  In the basic rules it says that attacking does NOT break concentration, that is a trait of the spell invisibility, so the Imp can attack while still being invisible as long as the party does guess it's location, hit it, and the Imp loses the saving throw to keep cancentration.  

The stat block doesn't reference the spell, but it does specifically say "until it attacks or until its concentration ends"

or you could wait until you inevitably find one in the campaign which isn't unlikely most dms I know love these guys and then coerce it into becoming your familiar now it can attack on it's own and it gives you magic resistance 

"Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll."

Basic Rules - Find Familiar Spell Description

RAW for the imp state "Invisibility. The imp magically turns invisible until it attacks or until its concentration ends (as if concentrating on a spell)"

As long as the spell being cast does not specifically state as making a spell attack, the imp would remain invisible while casting.

nice loophole

.

.

Quote from Mindseye >>

Do you have any source that indicates that delivering a spell is any different the casting it?  It's still being the conduit for a spell which breaks invisibility. The fact that it's healing, not an attack is irrelevant.  It explicitly says attacks or casts a spell. Now, he could administer a potion or a goodberry to get someone back on their feet without breaking invisibility.  

 It looks like instead of casting the invisible spell it's giving it the invisible condition with the caveat that if it attacks it breaks the invisibility the description doesn't say anything about spells I would understand if a DM said a spell would break the condition but it seems like rules as written it wouldn't

Hey Hey Hey 

Quote from Gay_Dog >>

With this or the Quasit as Pact of the Chain familiars, you can just keep them in their beast forms all the time if you want.

They'll still be able to talk, turn invisible, inflict poison, etc.

The imp actually flies faster when it's a raven!

R.I.P Bing Bong

Making a Thri-kreen wildspace pirate with Mask of Many Faces that always looks like a stereotypical pirate captain and has an imp disguised as a parrot :)

Question: so if a mortal soul gets turned into a Lemur and then that lemur gets turned into an imp after being promoted to a higher form of devil, does the imp get promoted as well, then becoming an even more powerful devil? My real question here is am I to believe that Raphael from bg3 for example, was at one point an imp?

It doesn't matter anyways.

Invisibility. The imp magically turns invisible until it attacks or until its concentration ends (as if concentrating on a spell). Any equipment the imp wears or carries is invisible with it.

Casting a spell doesn't necessarily break concentration, so non-concentration healing spells should work.

Quote from Gay_Dog >>

With this or the Quasit as Pact of the Chain familiars, you can just keep them in their beast forms all the time if you want.

They'll still be able to talk, turn invisible, inflict poison, etc.

The imp actually flies faster when it's a raven!

Any recommendations for a basic rules creature that could ask riddles (like a sphinx but easier to kill for beginners) 

The creature type options for a  familiar are fey, celestial, or fiend, and you can have a raven familiar as well. Detect magic wouldn't reveal that it's an imp. Though some people may think having a fiend as your familiar is a bad sign, morally speaking.

I call the imp sebastion