Congratulations Nick.
I'd suggest sticking with a single channel and keeping the squelch open like you're doing now. Some have reported good results with scanning but I'm not one of them! Single channel ops also makes it easier to figure out which channels are the most active in your area. Monitor one channel for 24 hours, record the results, and then move to another until you find the most active ones.
The number of position reports varies by channel and time of day. I reboot every night here and I'm running three scanners into the internal sound card and an external USB Sound Blaster sound card. In the past 21 hours I've had approx 5200 good decoded messages and only 205 positions plots.
Yes, it's normal to hear some tones that don't get decoded. I run with the ACARSD displayed volume around 12-15 or so with the squelch open and no signal. It peaks higher when a signal comes in.
Yes, using 10 decoder passes also increases the odds of decoding as long as it doesn't slow down your computer too much. On my Dell laptop (Pentium M 2.0 GHz) running three scanners into two sound cards, with 10 decoder passes, uses approx 20-40% of the processor.
Have fun.
-Dan