Any question answered...

Seatpost is definitely not slipping and I already smeared every surface in grease. I can feel a slight movement in the saddle when I accelerate or hit a bump, which is accompanied by the click. I'm fairly sure the round bit in this photo

is rocking back and forth. There are no serrated teeth. I think tightening the bolts helps but I'm nervous going any further as I have snapped bolts before.

The design is pretty shit IMO, but I have it and want to make it work.

I have this post, and also one of the original bolts is missing as with yours... you can buy replacements from pmp in italy for like 8 euros each but the fucking shipping is like 18 quid or something stupid. I very nicely wrote to them in italian asking if they could just chuck them in an envelope with a stamp on but they declined...
basically you need to file down the base of the bolt head where it would enter the recessed clamp hole to give it a curved profile, and then get a flat, thin alu washer, screw it tight so it gets pulled into the hole with the bolt head, and theoretically you won't die or get your ass impaled on a bit of sharp broken titanium.
edit

here they are 6 euros

http://www.pmpbike.net/pmp-bike-en/?product=vite-in-acciaio-per-reggisella-2-serie

Fucking minimum order of 40 euros and 18 euros shipping. bulllshiiiiiitttttttttttt....