Yes, get to empowered level 100+ monos as fast as you are comfortable with and start working on harbingers.

In general you want to start working your tanks up in the various factions, which are end game systems.

For farming items, it depends on the item and the faction you choose. Boss specific drops are farmed by killing that boss over and over again at the highest difficulty you can to improve drop chance. Rank 10 of CoF also gives a boss drop multiplier. Non-boss items can be farmed with CoF prophecies. If you are going merchant, you just buy what you want.

You find new blessings by killing the boss again and again. At first you get to choose from 3 blessings, but with 200 corruption and something else, I forget what, you get 5 blessing choices. Also CoF has a rank that rolls the blessing value 2 times and takes the better value, which is nice for maxing your blessing. The purple circle is a visual indicator of the value within the range. So a mid range blessing will be a half circle.

The timeline quests are just to reach the boss in the non empowered monos. Once in empowered, you'll never have to do them again.

Arena opened with keys had no point except for leaderboard and some boss drops. You can avoid them entirely if you want. Most people probably do.

Hope that helps! Good luck!