If I have a crystal matrix on a mage X, will he be able to establish a communion even if he isn't astral?
The crystal/slave matrix automatically casts Communion Master/Slave at the start of battle (before turn 1 in fact), so the holder's paths are irrelevant. They could be some super wizard, lowly pencil pusher or indy priest. They'll still be part of the communion.
Does it matter if slaves are casting their slave spell before or after X? I take it, the battle turn order follows the army list order?
Since Reverse Communions are no longer a thing, it only matters on the first turn. Obviously they need to cast Communion Slave to actually join it, so if you want to pump out a high-level spell on turn 1 they need to be before the Master in the list order or holding slave matrices. After that their army list order doesn't matter anymore. They'll just stand around and contribute no matter what.
I want to cast a level 5, three gem, spell on turn 1, X is currently level 3. If I have four slaves, I should be able to get to level 5 immediately, and cast it at a cost of three gems, right? If I have two slaves, X will be level 4, and needs 3+1 gems to cast the same spell?
I'm 60% certain that the master still counts as having level 3 magic and so can only use 3 gems total. Only 60% certain. It's definitely something you should test in a separate game, just to be sure.
Additionally, the slaves don't have the path of what is cast. Will X still get a level up in that path? How will it affect fatigue? In the battle spell queue, the spell I want to cast is greyed out, it doesn't seem to consider slave spells? What are the benefits of using a slave matrix instead? Less fatigue? Spell will get ungreyed in the battle spell queue?
The paths of the slaves only matter for reducing the amount of fatigue which they get, which can also be reduced with buffs or simply more communion slaves. The bonus from a communion affects all the paths which the master has, regardless of what the slaves have.
The spell is greyed out because the game doesn't know how big your communion will be when you actually cast it. As long as your paths are high enough during the battle your master should cast the spell. But there's also the possibility that they'll get ganked before then. Hence why it's grey.
The benefits of a slave matrix is the ability to cast the spell before battle starts and enabling non astral mages to be slaves. Neither of which are particularly worth the cost of a slave matrix. Getting a massive spell off on the first turn could be worthwhile, but you can achieve the same thing by using a crystal matrix and picking a master below all of the slaves in the turn order.
I'd like to know what the deal with communions is anyway. I'm not a very frequent player.
It makes a bunch of slaves stand around doing nothing, in return for a magic path boost for the masters and fatigue being spread around all participants. So it's good for getting high-level spells out or spamming mid-level ones all day. The path boost is log base 2 (ie: 2 slaves for 1 level boost, 4 for 2, 8 for 3, etc) but the fatigue spreading is not.