Okay, so both pieces of the grate your recovered are damaged, but with the silver from the jewelry a skilled smith or jeweler should be able to patch it without needing to melt down the entire thing (which, as you surmised, would wreck the existing enchantment). Silver could carry most of the power for the enchantment without trouble, but there are some holes in the enchantment's framework that'll need patching and filling in (think of teh enchantment is a sequence of runes around the grate's outer edge, where that edge is missing the enchantment is gone too).

You know of two ways to change the current enchantment. One is to selectively erase parts of the enchantment, and put in new parts there that do what you need it to do. This is easier, but a bit more resource intensive. The other is to convince the enchantment that its actually supposed to do what you want without changing the enchantment itself, which is harder but less resource intensive.

To explain the above a bit more, the enchantment is essentially a rune sequence with the instruction 'attract all salt in the passing liquid'. Method 1 is erasing the runes explaining 'salt' and replacing them with the substance you want to pull out of the water. Your replacement runes would be an integral structural part of the enchantment and would have to carry a decent amount of magical power. Method 2 is using the fact that salt has been fairly loosely defined for this enchantment (the enchanter was either not skilled enough, or didn't want to write what would amount to a three-paragraph essay to explain what salt was) to create an addendum that changes the meaning of 'salt' into whatever you need it to be. This'd be a lot more tricky, but your addendum wouldn't be part of the main enchantment (it'd generally consist of a small plate with a kind of sub-enchantment attached to the grate, and could therefore be made with far worse materials).

I hope that's somewhat clear, but I'll happily try explaining it more if it isn't.

For method 1 you're a bit short on materials. For best result you'd need to purify the diamond/quartz mixture a bit and use that to fix the enchantment on the grate, but that'd leave you short on materials to edit the enchantment. You could grind up some of the gems you've found, but you'd likely need those to function as the enchantment's power source. You reckon you could lay this enchantment in about 2 hours if nothing unusual happens and you have all the materials you need (not including time to physically fix the grates) (DC25 spellcraft to get it in 2 hours, 1 hour extra for every 5 below, half an hour less for every five above to a minimum of 1 hour).

For method 2 you have everything you need. You'd still need to purify the quartz/diamond mixture to fix the overall enchantment, but you could use the quartz left over for your addendum plate, and turn a section of the lead pipe you'd found into the plate in question. You'd reckon this'd take you at least 3 hours though to figure out how to 'word' your arguments for salt being something else (not including time to physically fix the grates) (DC30 spellcraft to get it in 3 hours, 1 hour extra for every 5 below, half an hour less for every 3 above to a minimum of 1.5 hours).

For both DC's, if you're less than 5 below the target, it'd take half an hour longer.

I'll also have an IC post sometime tomorrow.