I'm glad you're enjoying SynthNes. You can turn off velocity by unchecking the "VEL" checkbox in the channel group. The arpeggio editor for each channel can be summoned by clicking the "Edit" button next to the corresponding channel indicator in the "ARP" section (note the scrollbar). Also, you should be able to trigger the arp pattern for all channels at once by changing your MIDI input channel to 6! Let me know if you have any more questions.
And here is a video of the arp functionality I mentioned (multiple channels are edited at around 30 seconds).
