Interesting take. I see no difference in "When you do this on your turn..." and "if you do this on your turn..."
For example, consider these statements and the following questions from a normal everyday English perspective.
When you do X you can do Y.
When can you do Y? After you have done X.
If you do X you can do Y.
When can you do Y? After you have done X! (We all know how this works, "If you clean your room you can go play with your friends")
In other words, an if statement is equivalent in meaning to a when statement. Since it has been declared that the "if" statement doesn't specify timing. How can it be that the "when" statement does specify timing?
I think you are looking at this from the point of view of the in game fiction, while to me the rules specify the legality of player options. So the "if" in the SM feat simply tells you what is required for the bonus action to be considered a valid player declaration. In a sense it's similar to antidoping controls at the olympics: if you are found positive to the test within a given period around the olympics, you are disqualified. Whether the test happened before, during or after your race is irrelevant.
Anyway, regardless of my interpretation above, it seems that this difference between "if" and "when" is the intent behind the rules: JC twitted about SM that the shove can happen before or after the attack.
https://www.sageadvice.eu/2015/01/29/shield-master-feat/
while the SA compendium (v2.2 pag. 4) has
Does the “when” in the Eldritch Knight’s War Magic feature mean the bonus attack comes after you cast the cantrip, or can it come before? The bonus action comes after the cantrip, since using your action to cast a cantrip is what gives you the ability to make the weapon attack as a bonus action. That said, a DM would break nothing in the system by allowing an Eldritch Knight to reverse the order of the cantrip and the weapon attack.
Here JC is not specifically addressing the meaning of "when" and it's entirely possible that the difference between the tweet and SA is due to him changing his mind over time, but it supports the notion that they are using "if" and "when" to represent different level of restrictions on bonus action triggers.