Summons may be healed using normal curative spells (unless you are using some weird summoning variant that disallows that, but Summon Monster/SNA/etc. are all healable) assuming their type is usually subject to magical healing. (Summoned undead, constructs, etc. all heal or not as usual.)
Eidolons don't heal naturally but can be cured with normal healing spells. Summoners don't get those spells (because then they'd be healbots in addition to ruling in all other ways) but they do get Rejuvenate Eidolon - not because normal healing doesn't work on eidolons but because you don't want to give summoners general purpose healing.
Unless you find a rule specifically saying something is an exception, then they always fall under general rules. Everything is healable via cure spells unless they have a rule about themselves or their type or class or whatever that says explicitly that they are not.
Yes
As the text on Conjuration (Summoning) says:
bring manifestations of objects, creatures, or forms of energy to you (summoning);
They are not real things, and while that specific manifestation might be wounded, stressed or even dying, you can simply summon another manifestation of the same type of creature using another spell.
In theory, each casting of Summon Monster conjures a different creature from the last, otherwise, it would be impossible to summon 1d3 creatures of the same type.
James Jacobs said:
On Golarion, if you use a calling spell to conjure an outsider, and then kill it, it dies as surely as if you killed it on its home plane. If you instead use a summon spell to conjure an outsider, the thing you summon isn't real before and after the summon spell ends. It doesn't "go back" to an outer plane when you kill it or dismiss it or the spell ends... it just stops existing, just as it didn't exist before you cast the spell in the first place.
But that's on Golarion, Paizo's Campaign Setting, and might or might not be true on other campaign settings.
So, where the summons come from, what happens to them, what they eat and their personalities are explicitly left open for each GM to decide how to handle these things.
Most people treat summons like creatures made out of magic, that only exist while the spell lasts, while others have their own ideas for their home campaigns.
The only known way to summon specific creatures is knowing their True Name (from Ultimate Magic). The rules for that, however, are a little mixed up between Calling and Summoning outsiders, as they give specific rules to enhance Calling spells, but nothing is given about Summon spells.
But we can assume that the general idea works for both subschools, as no exceptions were given to Summoning spells.
So, for a generic setting, we can assume that you cannot summon a specific creature again (say, Bob the Eagle), if it died while being summoned. But you can summon any other creature of that same type.
Rules as Intended
James Jacobs also said, later, on the same topic (yes, it's a long topic):
When you summon a creature using summon monster or summon natures ally, how does it work? Does it conjure a likeness of that creature to fight for you or does it bring a real creature from somewhere?
It summons a "copy" of an idealized incarnation of the creature. A summoned creature doesn't exist before you cast the spell, nor does it exist once the spell expires.
That's the difference between summoning spells and calling spells. Calling spells DO conjure a real creature.
Note that, this time, he did not refer to Golarion when answering the question. Although he could be talking about Golarion when answering this. But this means that the intent of the spell is that you create a creature made of magic using summon spells, not real creatures.
So, even knowing the true name of a creature, it cannot be summoned with Summon Monster.
Best Answer
This is actually covered by implication in the Eidolon rules themselves, where it says, in part...
So, generally, in any case where the Eidolon is summoned after being killed, it will be on half hit points. This paragraph goes on to say that, if killed, it cannot be summoned until the following day. The Summon Eidolon spell is breaking this last restriction only.