[AccessD] Citrix Environment issue....difficult client

Well, yet another story of a contract-gone-wrong.

I developed a complex Excel 2010 XLSM workbook: 15k lines of code.
It references and calls a complex DLL addin from a third party....based on
the dot-net 4.0 framework.

Everything works well on a laptop or desktop environment.
Unfortunately, it does not work in a Citrix Server environment.
More unfortunately, the consulting company I engaged with had PROMISED to
their client this whole "mash-up" would work in Citrix.
There is much evidence out there that Excel 2010 has many technical issues
especially with large VBA code modules. The comments at the Excel blog are
telling:
http://blogs.office.com/b/microsoft-excel/archive/2012/07/18/introducing-exc
el-2013.aspx

My VBA is throwing errors when calling the complex addin functions under
Citrix.
I feel the problem is in Citrix or Excel 2010 or the complex addin.
The addin vendor claims their product works in Citrix, but they've only used
a "toy" (small) example in their testing.
I feel their addin is at fault here. Microsoft has ignored all requests for
assistance on this.

I am being "pressured" to "fix it" when in fact, I don't believe I can do
much.
It's getting to the point of harassment.
Advice ?
1) seek a lawyer
2) reneg the contract terms (very low rate right now) to get bigger bucks to
fix this mess
3) tell them to "take a hike" and walk away