Is there any side to side play in the sight? This is often a problem with many FALs. If so, you could try removing it and squeezing it (gently!) in a vise a bit. Otherwise, replacing the detent spring, as you mentioned, is worth a shot.
About replacing the rear sight assembly, you may have problems if going to a metric push button, as your current inch sight is a bit taller. If you get one of the folding push button's, you should be OK. Although I think on one the sight base rail is offset and on the other in the middle (forget which), so you'd probably be best off replacing the whole assembly and not just the aperature.
About the front site - if you have a standard metric front sight post with the taller inch rear, that's likely why you have to have it almost falling out. In this case, you may be better off just switching to the metric rear, which may solve all your problems.
I know that the izzy front sight posts are available in a wide range of sizes, but I'm really not sure what front's fit what gas blocks. The hodgepodge of parts the Century rifles were built with probably won't help the situation...
Regarding the trigger pull - you may consider switching to a standard FSE HTS. Most folks don't have a very high regarding for century's set. Both RSA and FSE are coming out with a HTS sets that are more of a "match grade" quality in the near future, however, so if you are thinking of upgrading, you may want to wait. Otherwise, you can always try polishing the contact surfaces and pins. Just be careful not to take off much material or alter the angles. Some folks also try to lighten the trigger return spring, either by replacing it, or cutting off a few coils. However, this can lead to trigger reset problems and would only help with a heavy trigger pull and not really do anything for the grittiness you're describing.
Rocko