The section on attunement is quite long, so I won't reproduce it here, but it refers exclusively to a "creature". It doesn't say anything about a player, a player character, a humanoid, or anything else that would exclude familiars from being able to attune to magic items.
The ring of spell storing itself likewise has no restriction beyond requiring attunement, so yes, a familiar can attune to it. I feel compelled to point out that some DMs might not allow a pseudodragon to use a ring because it doesn't have fingers, though.
As for concentration, the DMG says this about items that cast spells (on page 141):
The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration.
So if you cast a spell into the ring that requires concentration, when your familiar uses the ring to cast it, your familiar will have to maintain concentration, not you.
First, we will assume that your DM has deemed that the Telepathy of the Warlock is 2-way, as it was intended as only one way. You don't technically need the Great Old One's ability as Voice of the Chain Master allows this anyway.
Second, if you attack an opponent you can't see you would still have disadvantage, the only gain you would get with this combination is that you can help them target the correct location on the field of battle as per PHB page 194 and 195.
Also, you can do the same thing with Devil's Sight invocation instead and cut out the "Middle-Imp". It seems that you have a lot of redundancies and a DM, such as myself, could deem that you would be unable to point out but one or two of the occupied locations but not more as it would take time for the imp to communicate to you then you to your colleagues. This of course would have to be done using no visual queues which a kind DM would overlook but another could easily require int checks to help in targeting while interpreting the Warlock's instruction.
Bottom line is that your teammates still cannot see them so they still have disadvantage, they just won't have that auto-miss chance for targeting incorrectly. This all assumes the targets don't take the Hide action. There is a difference between unseen and hidden in the rules.
Additional note, the Devil's Sight that is granted by the invocation is better than the Devil's Sight that is present on devils, as the invocation allows you to see normally and the devils simply use their darkvision within it, which means they have disadvantage on perception based on sight.
You can see normally in darkness, both magical and nonmagical, to a
distance o f 120 feet. PHB page 110
Versus
Devil's Sight. Magical darkness doesn't impede the devil's darkvision. MM page 70
Best Answer
1: No, but you can use an object to carry the darkness.
The darkness spell targets a point in space or a point on an object that you are carrying or isn't being worn or carried, so an imp isn't a valid target.
But you could cast it on a pebble, then give the pebble to your imp and let it carry that, and you'd get the same net effect. The darkness moves with the object you've cast it on, so your imp can carry the object around and be surrounded by a cloud of darkness.
There is some disagreement about whether a darkness- or daylight-enchanted object can be moved by somebody other than the caster without messing up the spell, so you might want to check with your DM, but the general consensus (and my personal interpretation) is that the clause about the spell "moving with" the item it's cast on makes it clear that the intent of the spell was to create an item that somebody else can then carry around with them.
2: No, range only matters during the initial casting.
The rule here is found in the last line of the Range section of the Player's Handbook spellcasting rules (p.202-203):
3: Yes, concentration only ends under specific circumstances.
Again, in the Player's Handbook spellcasting rules (p.203), it tells you specifically what ends concentration:
And of course any other effect that specifically says it can cause casters to lose concentration.
Sharing senses is an action, but it isn't a spell and doesn't say it requires concentration, so it doesn't cause any difficulties.