Any party member who meets the requirements can use those items, not just the main character.
Companions can't use all equipments, they can't for shoes, armor, gloves and helm. They have their own armor set for that.
The only thing you can do about the companions armor set is to add to it 4 bonus items. You need find them for each companion as they seem each done only for one companion. But companions can use the equipment items that are 2 weapons, shield, belt, 2 rings and necklace.
I don't know why this design choice, they probably did the error to compare to other modern RPG that all use a single character so can't compare in equipment possibilities to DA2 allowing 28 equipment slots. But well it's been an error to compare to other RPG, for sure any player will compare to DAO or party RPG from the past.
The result of this is that many DLC items related to the armor set are difficult to use until a long time. For example the blood armor set when you play a mage isn't an option until late and if you choose mage a special mage build.
DA2's hard mode is certainly harder than Origins. I got through Origins on hard without much trouble other than a couple fights. In DA2 hard mode kicks my ass repeatedly and I have put it down to normal (which sadly is too easy).
If you ask me they need to nerf hard mode a bit, since nightmare is still there for the extreme challenge and normal is so easy.
About the difficulty, I also lost combats multiple time but a lot less than during DAO first half. The difference is DA2 system is close enough to DAO that I take profit of DAO experience when playing the fights in DA2, even if in fact many things changed. But when I lost a fight I force think and try find some tactics before to think lowdown the difficulty from Hard. I gave up only once for the mage against the ogre at the beginning, perhaps with the rogue too I don't remember. But I'm pausing the game right now because I lost a fight and woo I wonder is this one I'll be able beat it at Hard, it's the camp in hightown at night linked to a mini quest, still in first chapter.
That sort of thing is a pet peeve of mine. I see it quite a bit, especially in RTS games, where I can never seem to find a difficulty level that feels right to me.
Did you play Starcraft 2 by any chance? Normal was pathetically easy, but hard was too big of a jump in the later levels, at least for me. I ended up being hugely disappointed because I couldn't find the challenge level I was looking for. Warcraft III was the exact same way for me.
I don't play RTS since a long time and in fact never played much any.
I do agree that more than once I wish some games had more difficulty setup. But I understand design team are very hesitant to confront players with the choice among 10 difficulty levels. In case of DA2, the friendly fire option would have been a very good point to offer as a separate option. This would generate a sort of 4*2 ie 8 difficulty levels, too bad they linked it to Nightmare.
In case of party RPG it's complicated because of the party, this can involve so many different setup and it's impossible to provide them all the same difficulty level. It's also a problem when a very difficult fight force you optimize better the party, for example through an equipment improvement of the whole party, and after that you could end in too easy fights. That happened to me in DAO.