It's impossible to lose mats when a synergy fails, so no need to worry. ^^

Technically, it's supposed to be possible to complete any synergy recipe regardless of skill, but the odds are definitely against you. Stall recipes do require high skill to make easily, so your best bet might be to find a capped synergist to make it for you if you want it anytime soon..

Leveling synergy is entirely different than synthesis. The standard method for skillup for synergy is to find the highest level recipe you can get mats for and spam "Feed Fewell" for an element needed for the recipe but never finish it. (Stalls are actually perfect for this since they can take you all the way up to cap.) Since you don't lose mats, you only need one set of mats for your chosen recipe. In other words, you actually WANT to fail the recipes. lol You can expect a total cost of 2-3mil for 0-80 if you refill fewell at the npc. Maybe a bit less if you have access to a master alchemist who can make the fewell items for you, but generally it's simpler to just use the npc. It's a rather mind-numbing process, but can be done in a couple days.