The Autarch's buff is worth about 1/2 of a Guide to every unit it applies to. I think an Autarch is a reasonable choice if you can get his buff onto a couple of units, and also make use of some other aspect of his kit - ability to farm command points, counter-charge capabilities, etc. If you can get his buff onto *3* units that need it, then it doesn't matter whether the rest of his kit gets used, he'll give good value.
If your concern is whether or not the aura is worth the cost on Fire Dragons, the main comparison here should be, "what if I just took a full 10-elf squad of Fire Dragons instead of bringing an autarch along with them". In this case, 3 more Fire Dragons is probably going to have a bigger impact than an autarch buffing a 7-elf Fire Dragon squad would.
One thing to remember is that your Autarch's Path of Command ability only works on units that share a craftworld with him.
RE: running Farseer + Eldrad - I think this is pretty reasonable. I'd give the Skyrunner Doom + Executioner, and give Eldrad Doom + Guide + Fortune. In a typical turn, Eldrad will just Guide, Fortune, and Smite. The reason we also give him Doom is in case the Skyrunner dies, and also because it means that you have two potential Doom casters - you could have Skyrunner Smite + Executioner and have Eldrad Doom, for example, if you really want a bonus to Doom, or if he's closer.
Generally speaking, Eldrad is more than 1.5x as good as a single Farseer, and costs less than 1.5x as much. He's not a melee monster or a spellcasting beast or anything, but he's one of the more reliable casters in the game and has access to good spells. The only downside is that he's not on a jetbike, but if you position him carefully he is good value.