Try not to take it personally and just quit before the match starts. I have also started having fun with it and if their first pick is a +4 Destoryer of Worlds, I try to pick the craziest team ever, like all of my champions have to use a bow, or they are all christmas based. The Grinch, St. Nick, the snowman guy, and then usually the panda for some dumb reason. 

As a new account (3 weeks in), is Live Arena required, say at some point in Arbiter missions, or is this optional?

It's required for the Horsey (Marius the Glallant) missions.

Also for Quintus.

At 3 weeks in, I'd say you're probably too early to worry about doing live arena, but otoh you can't lose points until you get up into gold, so i recommend using your free tokens everyday, even if the only time you win is if an opponent quits

Only for marius missions.  You will struggle immensely, but nothing to lose but time. When you use all your energy and other available tokens might as well give it a spin. 

Roger that.  Got a lot to do before that.  Get Arbiter, get Ramantu...heck I'll even take doing 10 million to the Demon Lord. 🤣

demon lord is actually one of the very first things you should work on, it's a HUGE source of shards and books

@karcher 

Demon Lord is way more important than Arena, in any form, for your long-term progression in the game.    

On Demon Lord, sorry not trying to hijack thread, is Coldheart with Toxic a good means to up DL damage for now in early game?

Yep!