
Changing Man |
Any kind of alchemist bombing away with Holy Water would be lethal in this campaign, and the regular bombs would be great against any non-demon foes. And alchemists get both bombs and mutagens (unless you take an archetype that swaps out the mutagens for something else), as well as their infusions. So, yeah, it's more than do-able. Champions of Purity has some goodies for alchemists inside,
Regarding tinkerers, if you don't mind 3pp, there are some things out there from Interjection Games and I think LPJr has something as well.

tsuruki |

If you want to make a new diplomatic character.
Try a Cavalier or Paladin.
The cavalier can have fun with a moderate Charisma and his skills. Just use traits to turn skills you lack into class-skills and you're good to go to be the Public relations guy. With a suit of armor, a good mix of weapons, backup weapons and a shield you can do anything. Your bab will be high enough to use a bow and arrow effectively without feats. Your AC will let you take hits after diplomacy fails, and a variety of two-handed, sword & board, dual sword fighting will let you go wild with your Challenge damage bonuses and be effective all the way.
If the mount would get in the way keep in mind that the mount is basically just one class feature that you can definitely go without.
The Paladin can, along with everything stated above, also serve as a much needed healer. There are several abilities that let you effectively heal or even raise your dead allies using spells, feats and/or Lay-on-hands. The Alignment downside depends on your GM, if he is the stern sort that makes bad RAW calls on your alignment and instantly lashes you with his whip for killing a surrendered creature, even if that creature is a Succubus youre 100% sure is trying to fool you.

Changing Man |
@ Coridan - I think she meant 'Tiefling' and just made a simple spelling error.
What could a good Alchemist demon slayer look like?
Well, the Grenadier Archetype would be a good bomber route. You give up poison and the Brew Potion feat to basically become really good with bombing things and also giving your weapons some temporary alchemical properties as well. You'd need to check with your GM if s/he would also let you use the Alchemical Weapon ability with Holy Water; it's technically not an alchemical liquid, but it often acts like one.
Plus, you get to choose a Martial weapon to be proficient in, so your offensive options are better. (I'd advise upgrading whatever weapon you choose to Cold Iron; it's a small price to pay, and it's effective against most demons.)
As far as a good 1st level Feat, I'd go with Splash Weapon Mastery; it basically makes your bombs and thrown attacks even more effective. Later on, Ricochet Splash Weapon can likewise be useful (or you can reverse order, as you prefer).
That's just a few suggestions; Keep in mind you'll need a good INT to make your alchemy (and splash damage!) even more effective, and a high Dex can help with AC.
But hey, it's your character; do as you will !
A good early discovery to take is Infusion, since you otherwise won't be able to make potions (until you take the Brew Potion Feat) to share with buddies, then at 4th level I'd suggest Ranged Baptism, and later on at 8th Holy Bombs. Healing Bomb and Spontaneous Healing can likewise both be useful.

Changing Man |
A summoner's eidolon can look like a golem, but it wouldn't actually be a golem, if that's what you're going for. In other words, it wouldn't have construct traits and all that.
From the party composition you posted earlier, a summoner might not be much of a big help until you get to a level where you have enough Evolution Points to make your eidolon effective against demons (and their DR). You can get around some of the demons' DR with a level-dip into Druid & access to the Summon Nature's Ally spell, then the Starlight Summons becomes your new best friend.
But then you can also avoid any level dipping and take the First Worlder archetype, and then Starlight Summons as your 1st level feat, and you're good to go. It would also be a good thematic choice for a gnome.
But be advised: the party as you described has no healer.what.so.ever. And no access to Divine magic. at.all. With no healing power, the party won't last long.

Tangent101 |

You can just as easily handle healing with the Alchemist. And you can have fun while doing so.
Ultimately what you want is a class you'll have fun with. You seem to like the idea of the alchemist (especially by skinning the alchemist with "technological" devices that fall apart quickly).
One other possibility is if you have time, waiting for the Advanced Class Guide to come out. You might like one of the new classes better.
And really, if you don't want to do healing, don't. Your group can rely on healing potions and purchase a wand of cure light wounds eventually. And for that matter, you don't have a Paladin in the group. The Wizard can take the spell Infernal Healing and handle initial healing for the group.

Tangent101 |

That depends. Heck, you can even go with Witch and still do healing - in fact, Hedge Witches have the ability to turn any memorized spell into a Cure spell of the same level. Add in the Archmage talent to cast touch spells at range, and you can heal someone 10 feet away per tier. ^^ (Technically, Cure spells are touch spells and thus would qualify for the appropriate Mythic ability.)