Too complex? No such thing… (Need help commissioning the Duex4)

Robathome

I have acquired a Duet 0.8.5 controller and a Duex4 expansion board, with the goal of running a Kraken hotend (4x extruders/heaters/thermistors) on my Kossel delta, as well as a chamber heater, various fans, and a handful of external controls. With this in mind, I need help getting past a few hangups:

Can someone explain the Duex4's breakout pins for me? I'm reasonably confident that I know what connects to what, but I want to be sure… I think "H-" and "V_IN" are the +12V and -12V connections to the heater cartridge, "TEMP" and "GND" are for the thermistor, but the last three ("STOP", "3.3V" and "GND"), I have no idea...
Despite being the most in-depth and helpful wiki I've seen in a long time, the Duet3D wiki says nothing about which pins specifically on the Duex4 that can be used to trigger events, the same way endstops can.
I have 4 subsystems that I'd like to control through the Duet: A chamber heater (110V AC), mixing fans (12V DC), water pump (12V DC), and layer fan (10V DC). While the chamber heater will be controlled with a relatively forgiving solid-state relay (3V to 32V input), I need to know the output voltage(s) of repurposed pins in order to select relays to control the remaining subsystems.
Last week, I had a spool of filament that somehow tied itself in a knot, and to my surprise, the Select Mini hauled the spool AND the filament dryer it was in right off the shelf… I've seen an article on out-of-filament sensors and interrupts in the wiki, but I've never seen any "filament tension" sensors. In either case, is it possible to pause the print, wait for a button-press before resuming, and most importantly, beep at regular intervals until then?
While I intend to wire a status light (110V DC) for the chamber heater in series with the system itself (I accidentally bought a 110V lamp instead of a 12V one), I'd also like the Duet to control the status lights (12V DC) for the other subsystems. This means wiring the override switches to input pins, wiring the status lights to more output pins. Simplify3D has an option to insert a script that follows every retraction event, which is probably as frequent as I can hope for. However, I'm unsure how to use M581/582 commands to scan repurposed input pins, and if necessary, update the repurposed output pins.

I know this is a complex design, but it should be entirely possible for the Duet/Duex4 combo… I just need a little help with the details!

dc42

1. Correct. Stop/3v3/gnd are a set of endstop connector pins, exactly the same as on the Duet.

2. Any of the 4 endstop inputs on the X4.

3. All heater and fan outputs are whatever voltage you feed to the Vin terminals. There are no flyback diodes on the heater and fan outputs, so if you connect a relay coil, brushed DC motor of other inductive load, you must add a flyback diode.

4. I haven't seen any filament tension sensors either. We'll consider adding a "beep until acknowledged" option to the M291 command.

4. Use M581 to set up a macro to execute when one of the input pins changes state. See https://duet3d.com/wiki/Using_servos_and_controlling_unused_I/O_pins for how to control output pins.