only strange thing is the connection to the ignition, I was thinking of using the typical amplifier +12V instead.
why is it strange? sorry, it may indeed be strange, but i'm unclear exactly what you're referring to!
pin #8 connects to a 'switched' +12V wire to trigger the sub/amp on/off when you turn the car or headunit on. this wire can also be called: ACC (accessories), IGN (ignition), remote trigger wire.
shouldn't be too hard to find such a wire in either the factory loom, or piggy back it off the remote wire/terminal of your alpine amp (is this what you're thinking?), presuming this wire will have sufficient output-current to trigger 2 components (it should, depending upon where it is sourced from).
the 'constant' +12V powerwire (pin #3) will need to come directly from the battery, fused. if you are going to power 2 amplifiers, then a single main powerwire from the battery, ending in a suitable distribution block, then separate powerwires from there to each amp is the best way to go. for example, a single run of 8awg (american wire gauge) wire with a 60A fuse, going to a distro block (with multiple 8awg holes). then more 8awg from there to the alpine amp, and 8awg or maybe thinner, to the sub. the sub amp probably only need 12awg wire. this has a max current handling capacity of 30A, so you would also need a fused-distribution block (ie: a DB that holds inbuilt fuses), and add a fuse no bigger than 30A to protect the 12awg wire. this is cos the upstream 60A is too big to safely protect wire as thin as 12awg. alternatively, you could fit a smaller fuse near the battery (ie: 30A), and not have a fused distro block. would be ok so long as both amplifiers together don't draw more than 30A, which is probably going to be the case.
battery -> 8awg powerwire -> fused-distribution block -> 8awg to alpine amp + 8~12awg to sub amp
I don't expect too much from it as the speaker is only 6.5"
it's better than no 'sub'. cos it's in an 'enclosure' (subbox) and relatively 'long excursion', it's not too bad. more bass than your door speakers can produce.
I think on the standard HU setup the sub is linked to the rear speaker directly.
not sure. in my factory bose setup, if i fade the speakers towards the front (ie: fade out the rear speakers), the sub volume does not fade away. so it must be independent of the rear speakers. possibly connected via the front speakerwires.