1) Can the path changes (Summoner, Mage, etc, and probably associated resources) be used independently from the Factions?
Yes you could install the mod and then not use the new factions. Everything else would still be there.. so you'd still have the new spellbooks, resources (Faith & Summon Pool), some new monsters, champions etc. But it should work just fine if you just play with the vanilla factions. You should also be able to create a custom faction and use the new faction traits and it should be fine.
I was originally planning on making it all modular so you can pick and choose but it's just too difficult to do that.
2) Are there any other Mods that are compatible with this mod? Or mods that are "included" in this mod? (like snaking, outposts, etc)
It's got a customized version of DsRaiders AIPlus and parrotmaths MaulDebuff included (makes Maul not so overpowered).
Other than that, i'm not sure what would be compatible as I don't use any others myself currently. I'm fairly keen to try and get a bit of a known compatibility list sorted though, so by all means if you have some in mind, post a list of some specific mods and i'll go check it out and get back with details.
3) Do the Factions replace original ones?
Nope, you can use either/or/combination... they should be reasonably balanced. There are a couple of tweaks to existing faction traits that will affect the default factions though (actually I should put that in the change log) but nothing too major. Eg. Warrios get 50% discount on Training Yard line of improvements.