Cool that it's not dead, at least; I know I've been using it a crap-ton for figuring out how best to trawl through the various feats and etc. for good ones. Really nice guide; hope you can get back to it soon. I did come up with some ideas you could add to the unfinished sections, possibly...
I will say this; probably one of the best uses for a Synthesist Summoner is as a tremendously effective Tank. Even if you can't heal easily(at first...), you gain bonus Hit points based on your Eidolon's constitution when you go super. Why would this matter? Well, Fused Link is why it matters. Once you go 'below' the temp hit points provided by your Eidolon, you can be healed normally. Making them what they are; a buffer. Natural Armor bonuses are ridiculously easy to acquire, Spell Resistance and Fast Healing both help(particularly if you can get your GM to rule that Fast Healing restores the Eidolon's temp HP, as that would seem RAI), though Fast Healing is horribly overpriced. The effectiveness of it, even if it can affect that temp HP, is outstripped by Large and Huge by quite a ways, considering that you gain more for your Con score than any other class. Combine your now enormous HP pool with Extended Mage Armor, massive natural armor, magic items, (Greater) Shielded Merge, and your buff spells, and you have a ridiculously tough suit of 'armor'. Maker's Jump gets you where you're needed, and supersized CMD means you aren't going anywhere anytime soon. Ability Boost (Dex) works just fine for Huge Eidolons, getting them both AC and extra AoO if you've got Combat Reflexes, so it's actually actually quite useful, albeit the cost reflects that(if you aren't going Large for some insane/fluff reason you will have EP out the wazoo, though)
Of course, if you can't do anything with it, they'll just go around. Which is why you have Reach (manufactured weapons). And Combat Reflexes. And Martial Weapon Proficiency (Horsechopper). And Huge Size. Did I mention Stand Still? How about Bite and Trip evolutions, with Improved Trip alongside? With this build, actually, you're so dependent on feats, rather than EP, that Human starts looking better and better. Take the Mount evolution so your party Barbarian/Cavalier/Fighter/Paladin can ride you into battle(if you go for that, though, you might actually prefer to just slaughter things with Pounce and Trample to allow you to rip through your target alongside your buddy)
If you care more about being impossibly hard to kill, so your companions can use you more literally as a living wall, grab Divine Protection, to get Charisma to saves. The Evolution Surges stack with each other(but not themselves, if that makes any sense) so grab a lesser Rod of Extend and a normal one and go to town.
Evolution Surge is the single most versatile spell in our arsenal, (I think), because of the Minor, Major, and Ultimate Magic evolutions. After all, remember; the Evolution Surges stack with each other(but not themselves, if that makes any sense, so you could cast Evolution Surge and Lesser Evolution Surge and get a 2-point and a 4-point, but you couldn't cast lesser twice to get two 2-pointers) so grab a lesser Rod of Extend and a normal one and go to town. If you take Minor Magic on it's own(hard to go wrong with Vanish), you can have extremely easy access to the next tiers; Greater Evolution Surge gets you Major Magic (Darkness is good if you've got Blindsight and think you/your allies will be able to use the concealment, scorching ray can be good otherwise) and another two or three point evolution(See in Darkness, perhaps?), and Evolution Surge gets you to Ultimate Magic. Now, that might be fine and dandy, but consider for a moment; if you cast Evolution Surge a second time, to get Ultimate Magic, the use/day resets. It might be argued that if you're grabbing the same spell again, it doesn't, but if you grab a different spell, it's hard to argue. And while, yes, the spells aren't great, especially trading a 5th for a 3rd, if you grab a magic item with 1/day or 2/day castings of it, you're effectively gaining all of those spells with a shared 1/day or 2/day. Add to that that you can use it for nearly anything else that you get via Evolution, and it becomes incredible. Fighting ghosts? Grab Incorporeal. Underground? Get Burrow. I don't even need to mention Energy Immunity or Skilled.
With Skilled (Intimidate), feats such as Call Out and Dazzling Display, and Frightful Presence now using your Charisma score for the DC, and depending on how you view it, your HD(I would not go for that perspective, but it's a possibility), you can make a Fear build extremely easily. Intimidating Prowess and Huge size also boost up your Intimidate score very fast, especially together, and become utterly terrifying, just as you should be. Class Skill doesn't even matter at the amount of bonuses you're getting. Some of the Evolutions actually seem custom-built for synthesists; anything that relies on Charisma in particular.
Finally, I'll say this; if you somehow feel you can spare the EP, you could consider taking Elf instead, for a very specific reason; use Summon Monster, then summon your Eidolon. Nothing says you can't, and the primary handbook for Summoners mentions it specifically. At higher levels(where the favored class bonus gets decent...), this gets supplanted by Summon Eidolon and Extend Spell, though. The duration on Summon Monster makes this both feasible and potentially very useful, if you have some prep time and a GM accepting of this level of ludicrous.