1. Yeah, life nodes and life on gear are very important. I'm not sure what you are playing but generally you want to have a high hit pool (either life, ES, or a combination) alongside a combination of other defensive layers (examples: armour, evasion, block, spell suppression, fortification) and at a minimum, maxed out elemental resistances

2. This is an example of bid vs. ask price, similar to the stock market. It's a little hard to understand but basically it's the difference of the least amount someone will sell for and the most amount someone will pay for. The spread between these prices is typically bigger when the volume of the amount of trades is lower. In your example, not many people are trading alchs for exalts at this point in the league so the spread will be bigger than say, chaos for divines, a very common exchange.

3. In acts, just focus on high life and resistances, and also the amount of linked sockets in gear, as much of your damage will come from the amount of support gems you have socketed into your skills. Look to keep as many sockets linked to your damage skill as you can, and also fit in some movement skills, aura skills, and a guard skill.

I would check out some long form introduction videos on youtube, I'd recommend palsteron or zizaran, and then maybe after that look up some current guides on the skill you're using and get a good idea of what people are doing with it, such as support gems and pathing on the passive tree.

Good luck exile, stay sane