I'm going to work backwards to answer you
1. An SM58 is fine, honestly every big artist uses them live when people are paying thousands for tickets- no reason you can't use one in the studio
2. I would not record vocals/guitar at the same time- no reason to, recording is about performance - I wouldn't sacrifice it.
3. The interface would deal with more latency than headphones and monitors.
4. There's great monitors at any budget- I'd recommend the latest Maudio BX5 speaker, or Yamaha HS5's or HS8's whatever you can afford
5. Most people go overkill on headphones - I prefer bose for comfort but a $75 pair of sony headphones (mdr76something) are standard in most studios
6. So look at the Axe fx is for guitar, but the Universal Audio Apollo stuff is for modeling preamps, and compressors and modeling studio gear- The Axe Fx II/3 have some of those options too- I'd assume the AX8 has some of those in the cab block- but I'd say UAD stuff is great.
7. In theory/on paper SPIDF is great but the Fractal quality is so high I get better results from the XLR's with a good interface.
8. The Apollo is good because it has processing power built into it- that can handle plugins/effects faster and better than computers, super low latency and great quality preamps and a/d conversion
9. I think the appeal of apollo is it's replaced a lot of outboard gear- so I don't run any outboard gear- no need
10. I use an Apollo Quad, but a Twin is just fine
11. You wouldn't be able to record stereo fractal and a mic at the same time with a TWIN, but i don't want you doing that anyway
12. One of their rack units would probably be overkill
13. Focusrite- Scarlett/Clarett are great- but the Twin would give you more professional quality and the plugins they use in realtime called unison plugins will give you a million dollar studio feel for $500. And save/replace the need for outboard gear