Derek Jackson said:
Is a French conventional Naval victory possible?
Possibly if the French had competent Admirals and a navy that could rival the British Royal Navy. That, however, is very hard to achieve: the British clearly had the best navy of the time, even if the French navy performed quite well during the American Revolutionnary War. Furthermore, a good number of French staff officers had emigrated following the Revolution: you no longer had any competent Admirals in the French navy, or at least none of the scale of Nelson or Collingwood per comparison.
Having the French navy able to confront the Royal Navy would probably require a POD before the French Revolution itself.
Derek Jackson said:
Could France have used some other tech, steam or submarine to damage the Royal Navy?
I'm not even sure there were plans to build submarines nor if any country had ways of making one. Plus, we're not yet in the Industrial Revolution in Europe: it started early in Britain but it didn't really kick in until the middle of the XIXth Century in the rest of Europe.
Derek Jackson said:
ASB or impossible. No one had the tech to build an underwater tunnel at the time I think. Plus, given the sheer size of the said tunnel (even with the shortest distance which would be Calais-Dover), it would probably cause a bankrupt of the French state because of its cost.
Derek Jackson said:
Also what about human powered hydrogen airships?
The French were pioneers in balooning. But we were at the beginning of that science, and the first airships could barely take in one or two people. And the first airships were not always reliable: it would be a too risky gamble in the eyes of many. Plus, I highly doubt the French would have the funds to developp a fleet of airships, even more so considering they would need a fleet of Zeppelins...