If you don't recruit her the first time you see her, she's not part of your party - I have to imagine you can get her later, because that changes at least one of the major parts of act 2, though not in a big way.
As far as DA:O, there was a lot of cheese you could do. Planting Alistair in a door to tank and having people cast/fire around him was tits. My rogue archer got ridiculous OP late game, to the extent that when I did the Morrigan expansion I literally solo'd the entire thing on the highest difficulty.
But bottle necking was a legitimate strat, even if it was cheesy, and I guess that was the reason for the random waves of shit, to prevent that.
My first playthrough was as a spirit healer in DA2; there was something borked about her in relation to her gear. It had some kind of hp regen that stacked in a weird way with spirit healer passives, meaning that she couldn't die. She had to literally be immediately smoked by 20 spells at the same time, otherwise her health just rocketed back to full a second later. When I upgraded her gear I found that effect stopped, so she was in weird gear even at the end.
But for me most of DA2 was just kiting. Spirit healer/Force mage combo was pretty legit, since I was able to knock back, slow etc and essentially keep aggro. That one event in the Felicia Day DLC where you click the altar was no joke though on the highest setting. You had to micromanage the fuck out of everyone every single second otherwise they were running into a ground effect that would instantly kill them. The AIs aren't awful, but there's no setting that 100% does the whole, "don't run through shit that hurts you".