My  "solution"...

First of all, get the context : 

Consulting the diary of Ricardo Viñes, the childhood friend, to see if there is something useful (thanks again to  Nina Gubisch).
The letter is dated : the first of February, that is a Thursday. The concert is then occuring on Saturday 3rd.

This day, Ricardo Viñes goes to a concert of the Société nationale de musique, then goes to

"the café Critérion (St-Lazare Square) with Fabre and Fargue; there was Ravel, Sordes, the Morland etc. I came back by the train with the Bénédictus and Ravel."

No luck ! Viñes did not spend his evening with Ravel !

Then, keep on studying the context...

About Mrs  Russel,  I read in the private diary of  Marguerite de Saint-Marceaux :

[January 6th 1899] Nice dinner. Fauré play all the evening. A fine English woman, Mrs Russel, has come and dined.

note1 : Fauré had written and asked Meg [Mrs de Saint-Marceaux] to invite for dinner Mrs Russel, the very nice wife of the Londoner singingteacher. He explained "Being not able to receive her conveniently, I hope you could take her in for me ! She and her husband are fervent "Fauréists" in London. [...] And forgive me to make of your home a dazzling and exquisite subsidiary of the parent compagny [in French : Maison mère], the little convent !" (lettre non datée, BnF)

Is that Mrs Russel the woman accompanied by Ravel in 1906 ? It is not impossible : Fauré has been Ravel's teacher of composition, his nomination as the Conservatoire's director is linked with the scandal of the Prix de Rome (1905), when his pupil had been rejected. Soon after, he will support the creation of the Société Musicale Indépendante, born among the Apaches.

Finally, sit and think...

When it is ready, lift it out of the oven :

It could be Michel Dimitri Calvocoressi. 

This musicologist has Greek origins. That year 1906, Ravel had harmonized Greek folksongs. The cycle had been performed (we don't know when) with a lecture by Calvocoressi.

It would explain why Calvocoressi'mother should be consulted : as a Greek woman, she could know the tempi.

This letter would enable us to precise the first audition of Chanson du pâtre épirote, A vous, oiseaux des plaines, Mon mouchoir hélas est perdu et Chanson de la mariée. (Marnat writes, as Orenstein : 1905-1906)

That's all. 

And sometimes rather tiresome to elucidate.

If you multiply that by a thousand, you can grasp what we're owing to Arbie Orentstein, Marcel Marnat,  François Lesure...

(p189)