Note that this p'sak is dated from a month ago - before Amnon Yitzchak went to Rav Elyashiv. In his weekly 'Shofar' he also printed two other piskei halacha signed by Rav Elyashiv, both dated from 4 years ago. It seems to me that AY is simply ignoring the wishes of Rav Elyashiv and publishing what AY thinks Rav Elyashiv should say (taken out of context, to make it fit with his agenda).
Of course there is no guarantee that Rav Elyashiv actually signed this or any of the other piskei halacha either - the printers have his signature on file. But we'll ignore that problem.

interesting. I didnt pay attention to the date on top because I saw the handwritten date by the signatures.. I figured it didnt matter when letter was composed, as it was signed later..

"He says concerts are prohibited even if they are completely separate."

Ever hear of "keeping up with the Joneses?"
This is keeping up with the Taliban (or the ayatollahs)

The PS after the signatures needs clarification, next time someone happens to chat with Rav Amnon.

It says that not only are the concerts prohibited, but the songs of "these singers" as well.

Which singers?

All singers who have ever performed in front of ANY audience EVER, even with exclusively separate seating?

Or only those who continue to perform, even to separate audiences, despite the ban?

Regardless of what Rav AY says, every Jew listens to his own halachic authority. If Moshiach comes and issues this statement, then it's something else.

Isn't there a Halacha that forbids music in Yerushalayim ever since we don't have the Beis HaMikdash? At chasunas one is not allowed certain types of music/instruments.

Could Rav Amnon be coming from this perspective, and the extrapolating to ALL music and singers?

unlikely because he a) doesnt mention the issur of music in Jerusalem and b) doesnt limit his ban to jerusalem

There is a "MINHAG" yerushalayim about not using live music at weddings.

There is a HALACHA about not listening to or playing instrumental music since the churban beis hamikdash.
There are exceptions to this rule. Perhaps they think that the concerts are not an exception to the rule.

Chazoka of all pashkevilim is that all the signatures are forged, unless proven otherwise.

With all due respect to Rabbi Sedley:

They obviously put the later date on the bottom to clarify that this statement of Reb Elyashiv post-dates the lenient statement of Reb Elyashiv.

Furthermore, Reb Amnon Yitzchak wanted Reb Elyashiv to ban the music based on the inappropriate beats and genres.

This letter takes an entirely different approach of ostracizing music based on the actions of the singer.

In lomdus, this ban is not the new ban of Reb Amnon Yitzchak, but merely a reinforcement of the old ban on concerts.

(Otherwise, the Slifkinites would run to say: but how could these rabbis ban a song when they can't even read music?!)

Also, They could not have really meant to ban every song sung by every singer who has ever given a concert. For example, on Lipa's non-stop album, he devotes an entire track to the songs sung for gedolim when they come to chasunos. That could be awkward. We need some clarification.

I don't see what the problem commentator #10 might have with banning ALL songs that were EVER sung by ANY performer EVER.

True, many of these songs might have been sung for gedolim in the past, but that was the past and now is now.

In other words, perhaps THEY could sing it, but WE can't.