Filter is only ever going to go to MIDI note 36.
Accent can indeed modify ADSR depth via the flexis. It’s covered in the manual. (I use this in some of my patches.)
Up to the current firmware, the FI transmits MIDI only on its own channel so all external synths must be on that channel. (In the next update, you’ll be able to set separate internal and external channels.) You’ll need to enable MIDI output on each patch. It’s only enabled on the first four factory patches by default. This is covered in the manual.