There is a flurry of answers to be found in a related SO question. Though none, other that mathdotnet, fall at the intersection of symbolics (the kind of simplificaiton you are asking for above), lightweight-ishness, and accessibility on .Net.
I see you have already found the mathdotnet forum. Note a couple of its developers are SO users:
- Christoph Rüegg
- Joannes Vermorel (though ohloh tells me he's more attached to the numerics section)
That might supplement the support you ask for.
