My best advise is to have your lawyer go after the original stop since you gave him permission to look in your car. That was all based on his stop and I do not believe he had probable cause.
Also look into the "ex post facto" angle. You have had that there for a long time and never used it. Clearly it was there for you own self defense (which you also have a right to) and was perfectly legal when initially obtained.
Finally, as others have stated, check into the shelf life of the CS gas and whether it was any danger to you or anyone...and make sure you get a jury involved. Like speeding to the hospital, since no one was injured or hurt, most people will undertsand this (hopefully) and either no-bill it at the grand jury level, or acquit you at trial.
Best of luck my friend and sorry you ran across such a bad apple...felony indeed.
Thanks Jeff,
It is messed up, but hopefully I'll get it reduced to a misdemeanor with 6 months probation, and if I'm good, it will go away...