RA390 Captain reports autopilot becoming disengaged at FL400. Autopilot engage button was not pushed firmly enough to reengage and not confirmed on the PFD. Altitude and track deviations occured.

Narrative

At a cruise altitude of FL400 ATC advised traffic would cross overhead 1;000 feet above from right to left. The aircraft crossed exactly over us. The co-pilot was watching the plane fly over and disengaged the autopilot while leaning forward to watch the aircraft go over. The disengaged tone went off immediately I was watching the plane as well and it took a moment to notice why it disengaged. The co-pilot didn't say he did it just looked at me then after a few moments said it was him. It had surprised him as well. I reached up and pushed the autopilot button within 15 seconds of disengage but after pushing the button I did not verify on the PFD that it engaged. The airplane was trimmed for that altitude already so it seemed it was on. I then looked back out the window (side). Approximately 3 to 5 minutes later ATC gave us a frequency change. I noticed the airplane in a very slight descent and turn; ATC also noticed at that time as well. This was a Center Controller approximately 80 NM north of LSZH. I slowly started bringing the airplane back to where it should be. Probably too slowly. At this point we were 15-20 degrees left of course and 300-500 FT low. When the Controller asked why we were turning and descending the co-pilot didn't respond right away. He didn't know and as I was correcting I didn't say the autopilot was off. As soon as I noticed I should have said something to him. His delay in responding to the Controller then resulted in the Controller again asking what we were doing in a more panicked tone. The co-pilot I think was waiting for me to be back on heading and altitude to respond instead of just saying what we were doing. I should have responded myself as I was correcting when he didn't. My slowness to correct and his delay in response got the Controller worried something was wrong. There was another crewmember in back and I didn't want to just jerk it back on course at FL400. But I should have reacted faster. We apologized to the Controller when handed off and they said no problem. Everything would have been fine if I would have verified the autopilot was engaged. Not pushing the buttons hard enough on Proline 21 aircraft and verifying it engages is a common mistake I should have caught. The buttons do not illuminate in any way showing mode selected is activated. You must look at the PFD and verify which I didn't.