How do I get the chief to help spot rare collectables. I could not find this in the guides
As for the tablets - I missed some when I was cleaning up the rocks. My rock pile is all cleaned up now, So are they gone for good or will they show up later

If you click the "Tech" button, and click the "?" button under "Leadership", you'll see that it says:

Note that you will need to have Level 3 in Leadership before your chief will start this behavior when placed on the top of the path.

The tablets will continue to appear until you have collected them all, but you will never get duplicates. Once you have collected a piece, that piece will not appear again.

Good luck!

What does the chief actually do? Does spotting make the collectables show up, or show up on the map, or if I put a kid up with the chief will they run get something...? I've found collectibles every time I've set the chief to spotting, but I don't know if I'm doing it right - they're often neither rare nor new, and I don't know if I'm missing the one(s?) the chief is 'pointing out' or what.

And now I did a pointing-out and found no collectibles. Hmmph. I tried the map and wandering around to the collectibles locations, but nothing showed up (that I saw).

When the chief stops spinning it means that a rare collectable will appear somewhere on the map. S/he doesn't point to its location. It will be rare, but may well be a duplicate, one you've already found. The normal appearance of collectables isn't affected, so if you've found non-rare items they weren't the ones pointed out. It can be difficult sometimes to scan the whole map before the rare collectable disappears, especially as some tend to blend in with the background, which would explain why you probably haven't always found one.