Ok, I think there are a few things here:
First, yeah, I think we've adressed the question of whether it's "I'm playing a caster" that's the problem; it isn't, but the makeup of the group (somewhat coupled with how your GM is designing/picking encounters; clearly you're not abusing your spousal privledges enough!

has some issues.
Now, to the issues:
1. Yeah, your GM could make things easier on you. And, in fact, by offering to let you run a second character, he's making things a -lot- easier for you, as he doesn't need to adjust things nearly so much for a 4 person group.
2. Pacifist healer is, yes, going to deeply suffer in a 3 person party. First, you built all ranged, but with 3 party members (and only one of those melee), there's not a lot of ways to keep the bad guys off your back. Second, your key attack power, Astral Seal, gets weaker with fewer allies; fewer allies means fewer people to give a +2 to hit to when your Seal hits. Third, your Divinity power -also- gets weaker with fewer allies, as there's a huge difference between getting 2-3 allies in a burst with 4-5 allies and doing so with 2-3 allies. Fourth, the difference of having one party member who isn't mostly doing direct damage is much, much larger with a small party (where your party loses 1/3 of their damage due to your using Astral Seal or missing) than a larger one.
So yeah; I do think your setup doesn't quite work; you've got a leader who excels in large parties trying to explore a dungeon with two off-strikers who are better at clearing a room of weak enemies than they are at taking out single targets (or hits).
Ways I could see taking it:
1. Go trad. A pacifist cleric still isn't ideal in a 4 person group, but it's more workable, particularly with a defender who can hit hard (probably a fighter). Sorcerer and monk cover the controller role pretty well, so just adding a fighter would probably help a lot.
or
2. Go Stealth. so you've got a ranged striker, a striker who can cover ranged play reasonably well, and you. One way to handle this is to play a leader who's good as stealth (an eagle shaman with an appropriate background comes to mind), and refocus as a ranged party. Instead of trying to go toe to toe with the monsters, the approach here is to leave them wondering where the attacks are coming from, while the party blasts them from hiding, then the sorcerer and monk emerge to finish up the remains.