Shawn,
1st - As long as the autopilot is ON you are in ATT mode. OFF you are in analog SAS. The A/TRIM switch and the light within and WARNING indication are to advise of a problem within the A/P.
2nd - If you push the stick it will NOT automatically trim. Let go of the stick and it will go back to the previous attitude. If you push the stick and press FTR or TRIM via the 4 way switch it will trim accordingly. In UPPER mode if you OVERRIDE ALT,IAS,GA,V/S,ALT.A,G/S or LOC you will get a warning indication on the PFD caption for the respective mode.
Now there are a few pitfalls, Google G-SPAU and G-IWRC accidents.
One thing I am not so sure of is the removal of the GONG and warning indication with A/P DECOUPLE in the latest software version. It served 2 purposes, it let you know if the A/P decoupled intentionally or NOT!
The issue with G-SPAU has been addressed in later software as the bug does not default to "N" any longer on startup.
A small point but the "HEADING" control should be on the ICP with a knob to turn it and push the knob to centre it. Same as the COURSE button. The AP controller should just have an engage pushbutton for heading - a la A109.
As I said before the AP on the 135 is a bit of a kludge.
Wikipedia - A kludge (alternately, kluge[1]) is a clumsy or inelegant solution to a problem or difficulty. In engineering, a kludge is a workaround, typically using unrelated parts cobbled together. Especially in computer programs, a kludge is often used to fix an unanticipated problem in an earlier kludge; this is essentially a kind of cruft. A kludge is an ad hoc engineering solution, inelegant in principle but possibly elegantly pragmatic, from klug [german] meaning clever.
Too many little things to be effected by heat, vibration etc etc..................
Although it is getting better slowly.
Bladecrack - the squint in the FCDS is in the AHRU!