Upon re-reading the series you do see Heather is arguably the centre of this book from the very beginning.  She starts off as an optimistic cheerleader, and slowly builds a sense of responsibility for everyone she knows.  She lets tragedy wreck her for a while, and then she picks herself back up, and is pretty inspiring.

Just a shame Bill Mantlo decided to put her in the suit. There was no need for that.

I'd have to check, but wasn't it mentioned that Mac's suit was the only one in existence, and that Delphine Courtney merely duplicated the powers of the suit, and hadn't actually re-built the suit as that knowledge died with Mac?

JB has addressed this before; Delphine Courtney was a robot and her body was rebuilt so she could duplicate Mac's powers. She didn't have a suit. So there was nothing to "rebuild" after Jeffries dismantled her rather violently. 

They would have needed to build a Guardian suit from scratch.

Not that it's impossible but that's not what they say they did. The stated explanation has that huge logic flaw (above) so... 

Beyond that, her getting a suit and becoming a superhero didn't make sense on its own merits. Heather wasn't a fighter or a battlefield leader; she was more like a den mother, keeping the team together and tending to their needs rather than telling them "okay, Walter, fight that guy, Aurora, go left..." etc.

And for the record, I never found Mac boring. He and Walt were my favorite Alphans (and remain so). I could see the rationale for his death but it was NOT because he was boring.