I'm still utterly undecided on this - there's no way I have enough knowledge to do anything more than conjecture - so do pleaseunderstand I'm being a bit devil's advocate in coming back on your points, FW. I'm interested in testing the watertightness of any ideas for or against the ideas of mounds or a cult of removers.

"So many have no trace at all of any cairn"
"If this was the case then I would have thought that at least one would have survived intact and been found by now."

Not if it were done by people with knowledge of where all the sites were, and/or the cult was sustained over several generations.

That said, could Bryn Celli Ddu be regarded as one that's retained its mound?

"others only have a small area around them that it couldn't possibly have covered the whole structure"

Whilst one or two could have had wider ledges and suffered subsequent erosion (eg Kings Quoit at Maenorbyr/Manorbier), the fact of the space being too small for many of the monuments to have had a cairn is indeed unarguable.

But could it be, as I suggested earlier, that such tombs are later constructions done by the uncoverers of the older ones? The narrow-ledge ones I've seen are all earth-fast, which are supposedly later than the Carreg Samson style ones.

"The structural beauty of most also says to me that they were built to be seen."

This is in incredibly subjective judgement based on your modern and personal ideas of beauty.

Plus, future people could argue the same about our culture's exquisitely crafted constructions of finely polished oak lined with satin being built to be seen, whereas in fact they're coffins.

Covered longbarrows contain supreme architecture, yet the were buried.

"Several have been found that were entirely covered by peat. None of these have cairns covering them,"

This convincingly proves that any ideas about recent cairn-removal are nonsense. However, it doesn't discount the idea of the removal having been at some point between construction and the growth of the peat.

"If they were happy to rob stone from the portal tomb I can't see why they'd leave either the tomb or some of the cairn behind though."

If you imagine that the portal tomb was still a revered thing and the intention was not to destroy it but rather to assimilate some of its ages-old significance into the new tomb then it makes sense to leave the remains of the old tomb. Kind of like a new author having a quote on the front of their book from an author you've heard of to lend it cred, without it being an attempt to usurp or destroy the works of the established author.