Good observations . . . to which I would add that another reason Welsh survived was its relative isolation. Until King Edward came along, nobody thought the Welsh were particularly worth conquering-- the poor land and rugged terrain didn't lend itself particularly well to farming and the value of the coal underneath wouldn't be realized until centuries later.
As you are probably aware, Cornish, Breton and Manx still survive in the academic realm of ancient language departments.
Breton is still spoken in France, but the government does not try to help its survival and apparently the number of speakers is steadily declining. The language was brought to that part of France by people fleeing from Britain.
There are people who have tried to revive Cornish (maybe mainly in Cornwall) but I think they have to "fill in the gaps" with material from Welsh (like the dinosaurs in Jurassic Park which were recreated with the help of frog DNA).