I am de-lurking to give you some cyber moral support.
OP, you have acted with dignity throughout - until you sent the naked picture to the football team. But I understand why you did it. Just be careful that your need for revenge doesn't plunge you into a space where you breach the law on 'indecent communication'.
^If a message sent is grossly offensive, indecent, obscene, menacing or false it is irrelevant whether it was received. The offence is one of sending, so it is committed when the sending takes place. The test for "grossly offensive" was stated by the House of Lords in DPP v Collins [2006] 1 WLR 2223 to be whether the message would cause gross offence to those to whom it relates (in that case ethnic minorities), who need not be the recipients. The case also said that it is justifiable under ECHR Art 10(2) to prosecute somebody who has used the public telecommunications system to leave racist messages.
A person guilty of an offence under section 127 CA 2003 shall be liable, on summary conviction, to imprisonment for a term not exceeding six months or to a fine or to both. This offence is part of the fixed penalty scheme. ^
www.cps.gov.uk/legal/a_to_c/communications_offences/#an12
If one of the wives of the football team opens their DH's mail, you could find the police knocking at your door. That is the last thing I would wish for you at this time. There is enough trauma in your life at the moment without you facing any more issues because of a moment of weakness, anger and revenge.
You could have reported that indecent picture to the police and they would now be knocking on the door of your wife's boss/adultery complicitor.
I am glad that you and your DW have agreed to have some space to think. I am heartened that both sets of parents have been told. It signals an honesty and openness that is necessary to move forward to the next stage of your marriage (whatever that might be).
The fact that both sets of parents have rallied round and given the necessary space is laudible. It is not for them to sort out the issue; their role is to give space and unilateral support.
Like you, I can't understand how someone would make marriage vows knowing that they were having an affair and would continue to have an affair for as long as you didn't find out. And to have the affair partner in the same room is just a slap around your face!
I do suspect that the phone went into the river because of more damaging evidence than a message saying, "I love you". You already know that that they were planning a joint (double bedroom) trip in December, and then when you were going to work away again next year.
Whilst there are secrets and lies, there is part of a person which isn't available to their partner. There are plans for a 2nd, parallel life that don't include you or the DCs. Those plans signal disrespect for you, her current life and her children.
It is okay for someone to cancel a wedding if they have doubts - even after 2 children and many years past-history together. The taking of the marriage vows 3 months ago with the adultery-partner in the room is beyond forgiveness for me: particularly when they were now planning a December meet-up. That would be my deal-breaker!
But!
This is a forum for discussion. You will get different opinions.
It is your real life - and only you can decide the way forward.
I wish you the very best of luck and love in your life ahead. You definitely deserve happiness and a totally loving partner who puts you and your family above all else.