I'm not even sure if it's a simple dichotomy such as that (it might help to avoid the Angevin inheritence though. Norman Conquest is pretty necessary though as the Saxons weren't anywhere near as interested in expanding into Ireland and so forth). Wales was conquered very early on, and as loyalty was preferred over notions of ethnic settlement and so forth, once the big rebellions ended in the early 1400s, there really wasn't any drive to crush the Welsh Language, hence it survived much better than Gaelic or Cornish.
Indeed, this is despite the fact that Wales was centralised into the English legal system in 1536. There is, however, a direct correlation between the areas which were left to the Principality of Wales after the Edwardian Conquest and the modern areas of greatest Welsh speakers, possibly due to the fact that the Marcher Lords tended to impose a stricter ecclesiastical hierarchy to get more control over the populace. Ironically therefore, it would appear that a more decentralised crown or a more drawn out, piecemeal conquest of Wales would actually help in this case.
Generally speaking a heavily retarded reformation would probably help cultural unity, and for Scotland we probably a way to end Scots Law without causing the OTL resurgence in Celtic nationalism.