You get ordinary XP from the usual sources such as daily or weekly quests, maybe even some achievements. As for ‘traditional’ HS (Classic) — it might be both, but I’ve not noticed XP gains from solo content/Mercs — only those blue things were displayed, and I think I got gold only from completing dailies or weeklies.
If you play those modes — probably it’s doable, yes. I’m still not sure whether it would be worth touching them for some BG emotes or packs, which are useless to me, or a strange skin for Tyrande, which I don’t like much.
My experience with some of the previous legendary events requiring to play some modes that I don’t usually do (Duels, BG and so on) wasn’t a very fun one — I just wanted to get it over with.