Good cause you don't have to suspend disbelief cause she's the one who sends you the letter if you don't see her talking to the mage in whiterun, go back and read the letter that she put in the tomb when looking for the horn, its signed by a Friend as well. You will only get the letter (the first one after killing the dragon) if you have the Dragon Stone from Bleak Falls Barrow before talking to the Jarl's mage in whiterun (who asks you to go to Bleak Falls to get it). Now if you go to Bleak Falls Barrow After talking to the mage and return with the dragon stone you will see her there talking to the mage, and after dropping the dragon at the watch tower you will not recieve the letter.

I've tested it out on 5 characters, and the only ones to recieve the letter from "A Friend" after killing the dragon at the Watch Tower, were the ones who cleared out Bleak Falls Barrow before talking to the Jarl of Whiterun's mage and turned it in to him when you first meet him. The ones who did not clear out Bleak Falls Barrow before the mage asks you too , never recieved the letter "From a Friend" and instead meet Dalphine when see is talking to the Mage about his dragon research upon returning from BLeak Fall Barrow.

If you disagree with me please test it out yourself or if you need to see how to activate the scenes please message me and I will send you a more detailed walkthrough

I've received the letters on EVERY TOON who's done the first part of the MQ (6 of 6). Only one of those handed Farengar the Dragonstone before getting the quest to get it - the other 5 saw Delphine with him, and STILL got the letter after offing the dragon at the Western Watchtower. So you see, I've already tested it out - and I simply don't believe it's Delphine. YMMV, and that's fine.

i still say that it is Alduin.

who else benefits?

I'm not sure how he would have benefited, but since I killed him I've ceased receiving those letters.

each dragon powerful enough to have a name is a rival to Alduin.

how many named dragons have you killed?

I got a quest from the greybeards directing me to Rannveig's Fast. On the way there, I stopped off at Elderblood peak and offed the dragon living there. Had to go back to Whiterun to unload those damned heavy bones, and got stopped by a courier when i got there. First ever "letter from a friend"!

It tells me to go to Rannveig's Fast.

"You caused quite a stir when you used your shout at The Throat of the World. Not everyone is anxious for the return of the Dragonborn."

I suppose I must have some "friends in high places".

Yup, just a mechanic to get the player to explore important dungeons. Nothing more nothing less. Also, leads you to dragon priests if I'm not mistaken.