1) why can't I get the Holdfast to HOLD FAST!?
I have used a few types of epoxy putty, this being the one I've used for the last few years. It's been more-or-less like the other putties I've used in the past although maybe a little mushier at first than some I recall using a long time back. Anyway, I haven't built elaborate scapes with putty, but have used it a lot to stabilize smaller things like corals and small rocks. A few things I've found from working with it in case you haven't already tried them:
- Superglue gel by itself can sometimes work better than putty for frags and other lightweight things if that's all you're gluing, as long as you can get a few points of contact. However, you mentioned a sea urchin, and urchins will chew superglue away rather easily if it is exposed.
- The surfaces have to be both dry and clean for the putty to stick quickly and well. An example would be a dried piece of coral skeleton onto a brand new frag plug. Sometimes you can scrub a surface clean enough to get that result, but it's hard with LR or frags with mucky bases. Any sort of microbial or algal growth will mess up the epoxy sticking in a reliable way long-term even if it isn't obvious at first, so it is unfortunately common for people to putty LR and/or frags together and then have the pieces pop free due to the sorts of slippery bio-films that normally cover aquarium surfaces. Superglue seems less prone to that, so one common solution is to superglue the cured fitting to the rocks if it pops free or even to just mold the epoxy into the right shape, pop it off deliberately, and then do the superglue trick on both ends. I've done this a lot with more oddly-shaped frags and larger, heavier rocks/corals where superglue by itself didn't have enough contact points.
- Putty used by itself really needs to be able to make a C-shape around something or fill a crevice to hold well. If you can find a way to do that and brace the arrangement while the glue cures, it will hold great even if the rock is mucky at the time the glue is applied. It still needs to be under minimal strain though; the rocks need to either be light or already be able to balance well in their glued configuration.