Brutal 12/3 or 12/6 are the farming spots you should be aiming for.

See here for an chart of XP and Silver gains in Campiagn:

https://ayumilove.net/raid-shadow-legends-campaign-xp-silver-guide/

Additional XP boosts simply add to the time you have already. They do not stack up the bonus XP percentage.

You have two Elhain at 60? One properly geared level 60 starter (Elhain in your case) plus 3 "food" should be able to clear brutal 12.3

One important thing about Campaign XP is, that the amount gained is evenly split among the champions in the team.

If you use two level 60 champions, half of the XP is wasted. If you use a single farmer and three food champions, only a quarter of the XP is wasted.

Oh, and as for knowing whether your XP boost is on, that is the small yellow XP banner to the right of the rewards. If it is yellow, it is on, you can also click on it to see remaining time. If it is a faded and brow or grey (I cannot remember now), it is not active.

1st: Answer:

You should only need 1 Elhain level 60 to Farm Brutal 12-3.

Put 1 of your Elhain's in Lifesteal gear - Sample Video

In the video, The guy used Horrible Gear.

The substats in all his gear pieces where Flat Stat Garbage.

The guy in the video also used Atk % Shoes.

I would go with Spd Shoes.

2nd: Answer:

Click on the Yellow XP Icon

It will tell you how long it last till it expires.

3rd: Answer:

If you have an XP Boost going, It will have a time duration.

Lets say 2 days 2 hours till expire

If you was to activate another XP Boost which last for 1 day, The time duration will increase.

Instead of 2 days 2 hours ---> It will say 3 days 2 hours

must say That's a stunning piece of advice, :)😀