Aftermarket products can SAY they are tested to NHTSA standards...the problem is, there ARE no NHTSA standards for them. If you use them, you are going against the manufacturers manual and voiding your warranty.... and *possibly* reducing the effectiveness of the seat (extra stuff under the harness can make it very easy to keep the harness too loose, mirrors can detach and fly at the baby at high speed, or be something that a baby hits their head on in rebound).
I prefer to educate parents about why these products *may* be risky, rather than give a blanket "No!"...I'm much more concerned that the seat is installed tightly (no more than one inch of movement at the belt path, harness SNUG on the shoulders, chest clip at chest level, harness in the slots closest to the tops of the shoulders, seat reclined no more than 45 degrees), than whether there are aftermarket products in use.
As for side or middle...the middle is safEST, but the side is by no means UNsafe, so you are certainly allowed some leeway in that choice for your own convenience (I hate for moms with bad backs or high climbs up to have to struggle with the middle seat...I mean, what if you slip and fall and baby goes flying and you are knocked out? Ok, probably not gonna happen, lol, but could, right?)
Anyway, use your best judgement and use aftermarket products that are either specificically crashtested with your seat (Graco has heaps of them they say are safe), or as sparingly as possible. And keep your cargo contained as well as you can...a mirror is no more dangerous than groceries or a stroller flying over the back seat in a high speed rollover, for example.
But since there is SO much misuse with car seats, it's easier to give parents simple rules to follow, and "no aftermarket products" is easy to say and remember. If I ever read a story where the child died or was badly injured and the cause is some car seat add-on, I'll eat my words and pray for that baby. But for now, deaths come from gross misuse of the seat itself, no seat at all, or unsurvivable crashes (little car vs. mack truck, for example)
Whew, I've gone on a bit, hope that helps!
Julie CPS Tech and mom to 2 in seats
http://www.cpsafety.com/articles/RFAlbum/SarahMA.aspx