No, this will not work.
The description of the Divination wizard's Portent feature (PHB, p. 116) says:
You can replace any attack roll, saving throw, or ability check made by you or
a creature that you can see with one of these foretelling rolls.
An initiative check is an ability check so it is an eligible target for Portent. However, it only affects "a creature"; even though the DM rolls once for a group of creatures, this is a roll for each of the creatures individually, and the player can only substitute the Portent roll for one of the creatures. This effectively breaks the creatures into two groups of identical creatures - the one affected by the Portent and the rest who aren't.
Scenario 1: The mount is charmed.
If the mount is charmed, then it will probably fall. From the section on Flying Movement in the Combat chapter (PHB 191):
If a flying
creature is knocked prone, has its speed reduced to 0, or
is otherwise deprived of the ability to move, the creature
falls, unless it has the ability to hover or it is being held
aloft by magic, such as by the fly spell.
Hypnotic Pattern sets affected creatures' movement to 0, so unless the mount can hover or is being held aloft by magic, it will fall.
It's unclear how long falling takes, so depending on your DM, the rider may or may not get a turn before they hit the ground. If they do, they can use their action to shake their mount out of its stupor as described in the spell. If not, their mount will fall and hit the ground.
If the mount can hover, or is held up by magic, then they still won't be able to do anything, but they won't fall, either.
Scenario 2: The rider is charmed.
If the rider is charmed, things get a little ambiguous. I won't post the entire text of the Controlling a Mount section here for copyright reasons, but suffice to say, it doesn't actually mention under what circumstances a rider can or can't control their mount.
If your DM decides that the rider can control their mount while incapacitated (seems unlikely, but you never know), then the rider will still lose their own turn but can (somehow) direct their mount on its turn.
If your DM decides that the rider can't control their mount while incapacitated, then the mount will act independently as described in the Controlling a Mount section. In this case, the rider will either be sitting there while the mount does whatever it wants, or possibly be thrown off. This is all pretty much up to your DM to decide.
Scenario 3: Both rider and mount are charmed.
As in scenario 1, if the mount can hover or is held up by magic, it won't fall, so rider and mount will stay exactly where they are. But if not, there's nothing either of them can do except fall helplessly to the ground.
Best Answer
RAW:
The DM decides what order creatures act when the operate on the same initiative. Therefore, the DM could rule that an awaken creature still has it's turn, and can still wake up the next one (spending 1/2 their movement to stand up, and an action to wake them up).
So: Yes, provided all creatures are all within 1/2 their movement of each other, the DM could decide (as the question suggests) that the group could end the spell on each other if even one character isn't affected.
RAF
However, if you are a DM, consider what this looks like to your players if you do this. Your player just spent a resource and their action trying to take opponents out of the fight. You then made a ruling that reduced the effect of that resource to only lasting a single turn. If I was a player and a DM did this, it would make me feel like I was really fighting the DM, rather than the goblins that the DM has put in our way. This is already a problem at a multiplicity of tables, and does not need to be exacerbated.
As a DM, if I were presented with this situation, I would say that each creature wakes up with their action already spent. They could still stand up and move 1/2 of their movement, but they could not awake another creature. This, I believe, is a more balanced approach. It allows the effect of the spell to be more powerful, but does not trivialize the encounter.
However, I suggest that you run your NPCs with no more than 2-3 creatures per initiative order, and at least 2 initiatives for each creature type. This reduces some of the swingyness of combat. You will less often have situations where all members of one side go first, and the other side is killed before they get to act.