Has it been proven that the palace were leaking stories about H&M?
No, it's only been alleged, mainly by those who believe that everyone leaks all the time. Main evidence seems to date from the War of the Wales and Diana's assertion that she was being briefed against.
What rot.
Christian Jones' partner was PROVEN to have sold stories to the Sun. Byline Times (a highly respected investigate journalism outlet that normally covers serious stories about Putin and North Korea) published copies of bank statements showing that Christian Jones' partner had received two payments totalling £4000 from a bank account belonging to the Sun. Christian Jones was at the time the Cambridges' press secretary, and he was not fired despite the obvious ethical issue and conflict of interest of his partner being exposed as taking payments from a tabloid.
The relationship between Jason Knauf (the man forced onto Meghan, and the man assigned to be Thomas Markle's press handler), Melissa Touabti, and Jason Knauf's best friend since college Dan Wootton (tabloid editor) is well-reported. Jason Knauf is the one who gave the Thomas Markle letter to the the tabloids. Jason Knauf is the one who made the "bullying allegations" without permission or knowledge of those who allegedly had been bullied, then emailed a tabloid to tip them off about it (Dan Wootton has confirmed this). Jason Knauf was collaborating with the Mail on Sunday over the lawsuit - MoS lawyers accidentally copied an email to Jason to Meghan's lawyers instead - and Jason had to do an about face at the last minute to avoid perjury.
Will and Kate responded to Jason Knauf's laundry list of fuck ups, betrayals and leaks by making him CEO of their foundation.
Melissa Touabti was fired after being caught leaking to the tabloids and after being exposed for faking her CV, yet walked straight into a plum role as a live-in nanny for a Tory billionaire whose sister is a tabloid journalist. Why on earth would you hire someone so untrustworthy and have them live in your home, unless that was part of the deal the entire time?
A tabloid journalist known to be friends with Carole Middleton and known to be sympathetic towards Kate is the one who leaked the story about Meghan and Kate having an argument at the dress fitting, a private conversation that could only have been leaked by someone either in the palace or close to them. How else would the tabloids have found out about a private conversation?
Someone leaked the news of Harry and Meghan's plan to leave, forcing their hand. Who?
It's pretty obvious that people are leaking. Otherwise how would the tabloids even know half this stuff?