The chance of your ALDA leaking is slim, and may not affect performance drastically. What happens is the lower shaft seal doesn't always seal properly. The way to test is with a MityVac that can produce pressure (like the Silverline Plus 4050 kit from thetoolwarehouse.net). You connect the pump to the ALDA and pressurize. It should hold pressure, and only leak down very slowly at pressures below ~10psi. Above 10psi it should hold pressure quite steadily. If the needle drops rapidly, or you can't get it to hold pressure at all, it's leaking. You can just swap on another good ALDA, however it may not be calibrated quite right - you'd have to drive it and see. Photos of the ALDA internals are here:
http://www.meimann.com/images/mercedes/OM603_intake/
I still this this is NOT your problem though, I'd start with the other things mentioned - especially the ARV disconnection, pressure line to ALDA cleaning, valve adjustment, and checking timing chain stretch. Looks like you have your work cut out for you!
Regards,