To answer your question, I think it's partially to help farm Farmboy Luke, but mostly unrelated: it's their way to find out if more hard node chances would increase energy refreshes. Just like they recently added a third 100-crystal refresh for cantina node energy, supposedly for a limited time, but it has stuck around because it likely increased their overall crystal churn rate. I think this is a similar trial. They say it's for a limited time so they can retract it if they decide it's working against their revenue stream. But if it actually increases revenue, they will leave it in place.
For your second question, it depends on your personal goals. If you are in a strong guild and Territory Battles is being planned for, I would go with Veers and Hoth Rebel Soldier. TB is imminent and I believe it's pretty clear that those particular characters will have bonuses.
If Territory Battles isn't your thing or you already have those toons, I would go for FO and Resistance farming at this point. You have plenty of time to fill out those teams to be prepared for new toons, events, etc that will be coming along with Episode VIII. FO is a really fun team, and Resistance can tear through HAAT and GW like nothing else. I feel pretty confident we won't be seeing a ROTJ-Luke type event until well after The Last Jedi washes through, so I wouldn't be pre-farming for that just yet.
And then of course, other toons on the list have their own place. If you need a boost in arena, Darth Nihilus fits on most any team, and with Gear 12 coming, getting him to 7-star is a priority if you rely on arena strength. Clone Sergeant is a critical piece of the Princess Zody HAAT team if you need a boost in raids.
So uh, I'm guessing that didnt' help much. But enjoy the grind!
