Clyde cairns are generally orientated between N and SE with a weighting towards NE. Interpreting the significance of this is difficult since many long cairns are known to incorporate one or more earlier round cairns whose chambers can have different orientations. Earlier excavations did not necessarily pick up this multi-period feature and classed these chambers simply as lateral chambers which are generally ignored in orientation statistics. But they may show the original site orientation.
On national boundaries, yes the likes of Gordon Childe and, of course, Glyn Daniel were more visionary. They were pioneering the classification of chambered tombs and looked at examples across Europe when doing so. They were, I suppose, also influenced in this by the popularity of the migration theories at that time.
Later studies tried hard to emphasise local initiatives and produced classifications to match. But part of the baby went out with the bathwater such that, to pick up one of your themes, we do not get a view of portal tombs which covers Ireland, Wales and SW England.
Hopefully it can change but it won't be the archaeos who will do it since that sort of thing is no longer fashionable in their community.
You mentioned "European Megalithia". Did you mean just the study generally or were you referrring to something more specific - like a website???
