You should be able to finish the campaign before the season starts. If you follow one of the guides you might be able to get all of the altars too. Like anything it really depends on how much time you can put into it.

I can’t answer that. Depends on factors like how powerful your character is.

My druid got a perfect roll on Trample essence early on so on main weapon my baby druid was doing crazy Landslide-Trample-Petrify DMG to everything, with landslide essence on amulet.

You can skip optional quests entirely.

I accidentally didn’t do my Druid quest until ~36 because strongholds are no joke at 15.

Also there’s an option to uncap framerate for cutscenes that I didn’t notice until I beat Lilith 😮

About alters, I thought they only will apply to other characters on eternal realm? I can’t confirm myself, I was just assuming.

Edit according to reddit/the first campfire stream

“They actually just addressed it in the stream. Fog of war stays unlocked, Altars and everything renown related will stay for your new seasonal character too.”

I am sure it’s only the first two renouns that are shared. Having extra paragon points off the bat would be broken -.-