Exactly, so if you haven't gained it yet, how can you use it?
If you haven't
not gained it yet, how can you be denied the ability to use it? In practice though, you just shove the creature and worry about the action economy later. You don’t know if you can use a bonus action yet, so you’ll just have to wait and see.
You have to satisfy the condition (take the Attack action) before you gain the benefit.
The condition is that you take the Attack action
on your turn, so later, on your turn, when you do, you’ll be able to use a bonus action, which you can choose to take at any time during your turn because its timing isn’t specified by the feat. Therefore, you can apply it to the shove attempt you made.
Think of it like this:
If you take money out of the ATM ("the Attack action") today ("on your turn"), you can use a bonus action to go to the bookstore to buy the book you wanted ("to try to shove a creature within 5 feet of you with your shield").
Without getting the money out of the ATM, you can't buy the book you wanted. (Please, no jokes about using your credit card or ordering it on-line.)
I understand how your interpretation works. But even this sentence you've written can be interpreted to mean you can buy the book first as long as you end up getting the money later. Maybe there's a certain amount of cash that needs to be kept on hand for some other purpose, so the cost of the book will need to be replaced.
Without taking the Attack action, you have no bonus action to shove with.
I'm not suggesting you use a bonus action to shove a creature without also taking the Attack action on your turn.
As for the time-travel part, that is unnecessary. If you have reached the end of your turn without taking the Attack action, you never satisfied the conditions needed to gain the bonus action so you could not have used it, either.
I'm not sure what you mean by "time-travel". I'm certainly not advocating for either players or characters to travel through time. If all you did on your turn was to shove a creature, then you took the Attack action when you did so. In the case of Shield Master, "taking the Attack action" doesn't correspond to anything in the fiction different from "taking a bonus action" as long as at least one of your attacks is a shove.
On a personal note, I think JC's official ruling on this is silly.
Personally, I don't like it, but I'd still play with a DM who made such a ruling even though I think it's a bit misguided. It's a valid interpretation of the rules-text.
With TWF you can shove first with your attack and then if you have the Extra Attack feature, potentially make two attacks with advantage. You don't even need a feat to do this.
I think you're saying if you have Extra Attack, you can shove a creature with your first attack then get two more attacks (possibly with advantage) using TWF. Without Extra Attack, I don't think shoving a creature counts as making an attack with a light weapon, but I suppose that's up for debate.
There are many ways they could have worded Shield Master to avoid this issue and personally should have simple by removing the condition of attacking at all. The FEAT is what should give you the bonus shove action using your shield. Just make the first benefit of the feat this:
"You can use a bonus action to try to shove a creature within 5 feet of you with your shield."
Nice. Simple. Use it anytime during your turn you want so it can be offensive, defensive, or whatever. For instance, an Eldritch Knight could use the shove to push someone away and then cast a spell as their action.
This feature never needed to be tied into the Attack action to begin with...
It isn't just Shield Master. It's the way many bonus actions were designed. I actually think it was quite ingenious that they attached the Shield Master shove to the Attack action because a shove is a melee attack, so it's potentially very flexible, given the right interpretation. Where they ran into trouble, however, is with the eldritch knight's War Magic feature because a weapon attack isn't part of the Cast a Spell action, which is from where Jeremy Crawford's revised ruling comes.