Now, I'm no expert by any means, but I have been doing some studying on these sorts of things.
At this point in the game I would not add more rock of any kind. It looks a bit crowded as-is. You do want swimming room, and the amount you have may be restricting your water flow and allowing things to collect in crevices. Even with good flow, stuff can collect in the rocks, blocking the pores the denitrifiers live in and creating ammonia (which, as you know, becomes nitrate) since it's just sitting there rotting. You want to blow this detritus out of the rock so the filter can remove it, and so your CUC has a way to get to what's left. I personally would leave the powerheads on so the junk doesn't settle again before it can be removed.
Now, about the sand bed...shallower beds like that do not support denitrifying bacteria, and will probably not become anaerobic like a deep bed can, but like a deep sand bed it still needs creatures digging in it and eating what settles inside the bed. I learned in freshwater that if you use sand, it must be stirred occasionally to kick out the junk to be removed. Your crew should be doing this job for you since it's a reef, but if they don't dig they may be missing junk below. Copepods, amphipods, nassarius snails, dwarf planaxis snails, and (I believe) dwarf cerith snails are helpful for this.
You can put your macroalgae in the display, but certain creatures (such as my turbo snails, or algae-eating fish) may well consume it. Calcareous algae such as mermaid's fan or shaving brush are good choices if you choose to keep in in the display. Chaeto's great but doesn't last long in my tank XD
I don't think temperature's really an issue here unless it's excessive; however, I imagine it could cause a die-off of some inverts that come from temperate waters. Those decomposing could add extra bioload for a while.
Keep in mind you're not trying to blast the rocks apart, and there may be organisms you don't want to injure. It takes a little feeling out but I'm sure you'll get it.
Once you're satisfied that you won't get any more junk out without the use of excessive force, gently blow off any particles, especially of sand, that have settled on your corals.
