1) Yes absolutely, although many things will be similar in Sigma, there will be changes, its probably best not to g in totally blind.
2) I dont have a definite answer to this but in my experience it allows you to get right to the core of a build rather than dick around wasting skills. I personally feel youll like that.
3) The mod has been out for a long time, a lot of the player base has dwindled down. Plus i noticed a lot played to sell items for TG and then died off again for some reason. A more seasoned player might have a better answer.
4) 75% of the builds on here are all viable without having to find crazy OP gear, some moreso for farming, and others for uber boss killing. some are a decent blend of both. In theory every build is still playable, some just have much higher gear and charm dependencies than others.
5) Easy starters are traprat/treewarden druid, Summon necromancer/barbarian, or just barb builds in general. there is actually a guide on how to get every uber charm with T6 gear on a barb, showing that its one of the most, if not the most, versatile char. https://forum.median-xl.com/viewtopic.php?t=19061
Hopefully that helps a little bit.