Regarding wealth, I haven't had this problem because my group tends to be the "get your character raised"-type. Rather than the "He's dead, roll up another"-type. However, it seems to me that a simple solution to the problem is to simply stop giving out treasure if the PCs loot their fallen buddies. They'll be over by some after splitting up their buddy's gear, but the problem will soon correct itself, assuming you don't keep giving them standard treasure to compound the issue.
Regarding the PC in general, I'd suggest first considering your battlefields. Perhaps start setting the combats in larger, more accessible areas, so enemies can reach the sorcerer. NPCs aren't stupid, and indeed many of them (fiends, dragons, etc.) are quite intelligent. Even if the sorcerer is hiding at the back of the party they're going to start throwing the nasty his way. First rule of adventuring: geek the spellcaster.
Also consider fighting fire with fire. Spellcasters are some of the most deadly opponents (as my adventuring group is learning, having just reached 13th-level.) A single
Quickened See Invisibility +
Disintegrate can really ruin a spellcaster's day. And really, what sorcerer or wizard is
not going to be familiar enough with
Fly +
Invisibility to not prepare a counter for it? It is, after all, the oldest trick in the book.
Besides a single BBEG spellcaster, even mook spellcasters (standing behind their own meatshields!) casting area
Dispel Magic multiple times can thwart the sorcerer's "hide and summon" tactic. A single potion of
See Invisibility and a archer readying an action can ruin his flying and spellcasting, as well.
These are reasonable precautions that might show up from any villain group without fear of metagaming. After all, your sorcerer isn't exactly treading new ground here. He's using tried-and-true sorcerer tactics. But those are exactly the kind of tactics that others are going to have ready counters for.
If you still feel that this is metagaming, consider having a couple of the bad guys
run away in the next few fights. Once they inform their master(s) of the group's fighting abilities, tactics will change. In the game I currently play in, for instance, all of us have
rings of fire resistance of varying strengths. We weren't careful and allowed an enemy to get away, and now certain foes (sent from or a part of that villain group) prep electricity or cold spells instead. The rings are still useful, but they don't completely negate the enemy, because the enemy expects them.
You could also consider using Outsiders. You're getting to the point where you can throw some really nasty demons and devils at them...creatures smart enough to know to geek the spellcaster first, and with the abilities to do it.
Consider a
glabrezu. CR 13, and with
greater teleport at will, as well as
Power Word: Stun 1/day. Once he identifies the sorcerer, he's quite smart enough (Int 16) to decide to take out the spellcaster. One
Power Word: Stun +
Greater Teleport and he's in a position to
tear the sorcerer apart.