Re: 6 month old peeing every 15 min
YES she will pee much less frequently when she's older.
NO you're not doing anything wrong. No one, no one at all, catches 100% of newborn pees, and very very few catch 100% of a 6-month-old's pees.
I can't say if she will potty train early for sure, b/c you always hear of people ECing full time from birth and having a 3-yr-old in dipes, but I can tell you that my 18-month-old is dry 95% of the time during the day and has been for a few months now, having EC'd very casually from about 5 weeks. Currently I offer the potty about every 1.5 to 2 hours (she occasionally asks for the potty as of recently, but not dependably), and she keeps herself dry in between. I WOH 3 days/week and the other caregivers didn't do much at all in the way of EC for the first 5-6 months, nor did my DH when he was alone with her, and even when I was with her I wasn't doing it all the time, only when reasonably convenient.
So I always tell people that at least in my experience it's still worth doing even if you're going to only do it part-time.
Also, I think your approach is good (at least, it's the same as mine, lol) b/c I think it's better to make pottying a positive experience (i.e. not spending half her waking hours on the potty when she's clearly tired of it) than to try to catch everything. It's such a relief and pleasure to find that my toddler and I have NO power struggle over the potty - she knows it's 100% her choice if she wants to sit on it.