In that case I think Spring is excellent for your immediate aim. Your ultimate goal should also work.

1. Spring games have models and they can be made in Blender or L3DT. Hopefully someone involved in modelling could say a bit more about the exact features of Spring for models.
2. You would use C++ to mess around with the engine and Java only for lobbies or AI. Most specific gameplay related coding is done with Lua which is similar in syntax to C++ or Java.
3. Spring has support for whole team AIs and several AIs have been written. You can write in almost any language you please including Java and C++. I recommend Lua for the greater engine integration but I'm sure others would recommend other languages. Formation based AI would be best done with Lua script ingame. Pathfinding would probably involve messing around with the engine source in C++.
4. Game balance is extremely easy to change and there are many behaviour tags to do so. Lua scripts can add most rules that are not in the engine and there are already a lot of lua scripts around if you want to focus on the effects instead of the coding.
5. Spring could be a bit too complex and undocumented to use as a networking tutorial but I'm no expert here.

blender for models
l3dt for maps

but yeah, what the frog said

if your modelers are less experienced wings3d may be easier.

I would not have said that. Yes, both Lua and C++ are programming language, but apart from that....

Lua is a high level language that is rather easy to learn and work with, but IMO the syntax is different from C: No semicolon, no braces, "then" and "do" after the "if" and "for", 0 is considered true, no manual allocation and deallocation, etc... See http://www.lua.org/manual/5.1/ for details.

semicolon is relevant in lua

This. If you want to compare Lua to a commonly-known language, I'd point to Javascript (dynamic everything-is-a-hashtable, implicit semicolon-at-end-of-line) or Python (ditto), or Basic (1-based arrays, lots of English).

+1

Lua is easy enough to learn but don't go comparing it to static typed non-functional languages. :)

Lua is more like a pascal syntax.

C++ syntax is similar to PHP is similar to C# (php of course being the uncompiled language of the 3), etc.

Lua syntax is similar to Java if you think about wildly different syntaxes such as Lisp or Prolog.

syntax, which is something like whether to use brackets or indenting, is very insignificant, if you think about how long it takes to switch from one language to an other. syntax just does not matter, practically. what matters is the type of language and the basic features (procedural, OO, functional, logic, ...) and the base functionality and common libraries.
whether java and lua have similar syntax is totally uninteresting. what takes long to learn lua for spring, is to learn what functionality is available (and ok, one has to learn to use which kind of loop).
(someone already said this in here)