Was it logical by Stuart to proclaim help to calabrian guerrilla hen he knew he could not do it ?

Logical or not, General Stuart issued three proclamations urging popular uprising against the French and direct support from the British army. He was the commanding General at the time and he had the absolute authority to make these proclamations.

It's easy to disembark weapons, money and some agents. It's another matter to provide remote basis, and eaders, to organize and strengthen popular fighters so that they can resist to the counter strike.

You are not looking at the strategic situation of Calabria. After Maida, the entire Calabria, including all the key strategic points on the west coast were in British/guerrilla hands. All of the strategic points in Calabria would fall soon after Maida. So, shortly after Maida, all of Calabria was free of French forces. Counter strike: sure but with all the strong points under their control and commanding all of the seas, the French would be up against one tough nut to crack. There were virtually no roads in the region and with British Navy total control of the coastlines the French would have a lot of trouble.

For sociological reason, he would not get support of his fellow british militaries to encourage irregular war, nor of Two-Sicily elites to organize social disorder. Excess of Sanfedist were not forgotten.

Just not accurate. Fox came in with the expressed idea of keeping the English Army strictly in Sicily.

For military reason, the support would obviously be discontinued, when no holding on Italian mainland, no strategical victory could be expected without a full scale regular army campaign that could not be fought there.

Again, just not accurate. Probably the biggest factor for the English army generals is that they had never beaten any Napoleonic army, other than at Maida of course, so, they (Fox) just wanted to stay on the big island and wait things out.

Why to encourage the revolt, knowing well that promised help with not come or will be withdrawn, leaving Vendéen, Chouan, Calabrian, Tyrolian alone to face repression ?

In Calabria, the repression came no matter what the decision of the English generals. The Calabrian guerrilla war continued until 1810 with 10s of thousands of peasants (Men, women, and children) being summarily executed, villages burned, etc, etc. Read "The Most Monstrous of Wars" by Finley, before trying to comment on what went on in Calabria.