I'm in the same boat as you with regards to whether EPA/DHA has a bigger effect than ALA, but I was convinced enough to try to find some when I became vegetarian last year.

If you google "algal dha together" you'll find what I'm taking - meeting your criteria of vegetarian (vegan), eco-friendly and health-friendly (with aforementioned uncertainty)

ALA can also be found in flaxseed, soy/tofu, walnut and pumpkin, so you needn't stick to seaweed if you only want ALA.