Okay, Echatl Nine...

[KaedrianLiang]

...just making them consumes all of the durability...

You're doing it wrong. Please consider other strategies while crafting these items. Search for guides or improvise.

[Nyalia]

Just for the sake of things, here are the level 15 cross class skills and how helpful they are in relation to the Ehcatl Nine quest items:

• CUL 15 (Hasty Touch): If you are low level (only ~10 levels above the official recipe level), this skill is practically required. It does the same thing as a Basic Touch, except the chance of success is 50% instead of 70%, and it costs 0 CP instead of 18 CP.
• ALC 15 (Tricks of the Trade): Extremely useful. It turns Good conditions into +20 CP. Since Good come up about 1/4th of the time, this will give you, on average, 40-80 extra CP. That's a lot of extra CP.
• LTW 15 (Waste Not): Useful for nearly any synth, especially these. Master's Mend costs 92 CP for 3 extra durability-consuming actions, for a cost of 30.67 CP/action (and Manipulation costs 88CP for 3, or 29.33 CP/action). Waste Not costs 56 and gives you 2 extra actions, for a cost of 28 CP/action. More importantly, sometimes you can't afford to pay for 3 actions, but could afford to pay for 2. Having Waste Not gives you that option. The only caveat is that you can't do anything but spend Durability for the next 4 actions after using Waste Not, or it's wasted, with one exception. One time during the synth, you can do just 3 durability-consuming actions following Waste Not and still get +2 actions. This is because there is no mechanical difference between 10 CP and 5 CP, or 0 CP and -5 CP. Because of this quirk, the limitations are effectively negated for one use of Waste Not.
• CRP 15 (Rumination): Useful with Waste Not, difficult to use usefully otherwise. Rumination consumes Inner Quiet and restores CP based on the number of Inner Quiet stacks (a fair average amount to use in analysis is +45 CP, which is from IQ6). However, Inner Quiet grants a massive quality bonus. Losing Inner Quiet before the final touch moves makes those last few touch moves about half as effective. This loss, in many senses, more than negates any bonus you get from having extra CP to work with if you plan on touching after Ruminating. However, you don't have to touch after Ruminating to get a benefit from having extra CP. The trick is to save a Durability increasing move and 2-4 Progress moves for the very end. Either: do all the touching you can, leaving yourself with 5 or 10 durability, about 47 CP, and the need for 4 more progress moves. Then, use Ruminate to restore your CP to 92, use Master's Mend, and then the four progress moves. You've just spent 47 CP for 3 extra moves, which is a great deal (15.67 CP/action). However, you need to have 4 progress moves remaining for it to be worth it, and that's not normally the case in my experience. Alternatively, if you have Waste Not, you can leave yourself with 10 durability (not 5), about 11 CP, and the need for 2 more progress moves. Ruminate, use Waste Not, and then do the final two progress moves. You've just spent 11 CP to get 1 extra move, which is an amazing deal.
• ARM 15 (Rapid Synthesis): Rarely good for Ehcatl Nine quests, but can be nice in the right circumstances. You do not want Rapid to be your finishing move because it has such a high chance of failing (same chance as Hasty Touch). You do not want to rely on it, you want to do it first if at all, during your Hasty Touch phase. Many Ehcatl Nine synths will be finished in a single use of Rapid Synthesis, making it useless for them. However, if the synth requires enough progress that a Rapid Synthesis won't instantly finish it, using one early on can really help you out later. In the case where you will need 3.5 Basic Synthesis uses to complete a synth, using one Rapid Synthesis early on gives you a 70% chance of saving 20 durability, which is a lot of durability.
• GSM 15 (Manipulation): Saves you a bit of CP (4-12), but that's it. Manipulation is an alternate version of Master's Mend. Master's Mend costs 92 CP and restores 30 durability. Manipulation costs 88 CP and restores 10 durability a step for 3 steps. The biggest advantage to Manipulation is that in 40 durability synths, you need to be down to 5 or 10 durability to get the full benefit from Master's Mend. You can Manipulate at 15 or 20 durability and still easily get the full benefit. This flexibility can be nice, but for other synths, it just saves you a tiny bit of CP.
• WVR 15 (Careful Synthesis): Can be nice to use in the right circumstances, but it's really not needed at all. Basic Synthesis is 100% progress with a 90% success rate. Careful Synthesis is 90% progress with a 100% success rate. But, 90% success is plenty. You likely will have some Steady Hand left over after you finish your Touches, making Basic Synthesis more useful than Careful Synthesis anyway.
• BSM 15 (Ingenuity): Totally useless for this specific purpose. Ingenuity lowers the level of the craft, which helps immensely for crafts that are a bit higher level than you, and helps some for crafts that are your level. However, the Ehcatl Nine quests are technically very low level for their difficulty. It is very strange for someone to attempt them with a class that isn't 10 levels above the technical level of the craft. This level difference results in Ingenuity having nearly no effect at all.

Hope this helps someone!