#2
While you can do it, I wouldn't. First of all, the headphone output from an amp is not going to be a very large signal - in other words, it's not strong enough to drive a loudspeaker. Also, you run the risk of an impedance mismatch. Most headphones are rated higher than 8 ohms and your PA speakers are likely to be rated for 4, 8 or 16 ohms. Not sure how tolerant it would be of this mismatch, but it would suck to find out that it wasn't and you smoked the headphone amp.
#5
If your amp has a speaker emulated line output (not sure if the spider does or not tbh) you could plug it directly into the PA via a line input, but not the PA speakers, because speakers take high voltage signals that a headphone output does not supply.
Live Rig January 2018

Fender Baja Telecaster/Danelectro DC-12
Boss TU-3, Ibanez TS-9, Fulltone OCD 1.7, EHX Small Clone, MXR Carbon Copy, EHX Holy Grail Neo
1964 Vox AC30TB with '69 Rola G12M "blackbacks"
Elixir Nanoweb 10-52 strings, Dunlop Jazz III XL picks