At a guess (maybe I'm wrong) but your players didn't want to start at level one, so they started at a much higher level closer to 10th. Doing so is fine, if everyone knows what they are doing and how to play their character, which by your description they don't. So it was very foolish to allow them to start any higher than level 1. Your players need to learn the game before they can effectively play it.
You say, your summoner PC feels like the eidolon is doing the work, but not the summoner himself. First of all, a summoner is a spellcaster, so is he casting any spells not related to the eidolon? I'd hate to see that player running a caster that doesn't have an eidolon, like a sorcerer or witch, because if he is a spellcaster and not casting spells - something is seriously wrong. Regarding the eidolon, it cannot exist without the summoner, in a very real way the summoner and his eidolon are two halves of the same character. Without an eidolon, who'd want to play a summoner? So if the eidolon is doing well, as long as the summoner casts a non-eidolon spell now and again - its as effective as he can ever get.
While my adventuring crews always include a cleric, if not some other dedicated healer, have your group ever heard of a Wand of Cure Light Wounds? Its probably the most common magic item in every adventure party. If you don't have a dedicated healer in the party then it is implicit that you have potions of healing, wands of CLW and other objects that can heal the party.
Have you considered adding a healer NPC to the party, someone who they meet on the road who will provide healing for an immediate need, and then volunteers to join the party? You could even use an Adept who stands no chance of succeeding in combat who waits outside the area of combat, waiting for an injury to call his attention and do his job. A GMPC is possible too, though many GMs don't play party NPCs well. A GMPC is an extra PC run by the GM for the sole purpose of party healer, in this case.
While its a definite requirement that your players learn how to operate their characters effectively, if you are still having party problems I suggest the real problem is the GM, not the players. You, the GM, aren't designing your encounters, nor altering existing encounters to accomodate the makeup and strengths of the adventuring party - this is your most important job as a GM. Every gaming group in existence generally cannot handle a published adventure as is, because often the published adventure has prerequesites of a full party of effective players that match the number and CR values necessary to meet the challenge of the module. I've never met a single GM that hasn't had to alter every encounter in a published module to accomodate the adventure party.
While adventuring parties built for most standard adventures generally require someone to take the post of the four basic classes in the game: cleric, fighter, rogue, wizard. Of course the variety of classes available allow you to take different options than those four basic classes, so these can be replaced with oracle, barbarian, urban ranger, and summoner, or some other combination. If you are playing a standard adventure, the four basic jobs of the adventuring party need to exist. If there is no party healer, you are missing one of the basic four.
Consider that I usually homebrew and create my own adventures, but some of the more specialized settings and adventures I've run were adapted to include a party of nothing, but rogues (all from the same Thieves Guild), or nothing but clerics, and such a party can be very effective as long as the GM is doing his job in building his encounters to accomodate the make up of that kind of group. So bizarre mixes and unequal balance of PCs should not be a problem. If it is, the problem is almost always the GM, not the players (unless your players really don't know how to run their characters, then that is the first priority in resolving the issue.)
