Hi -
We are facing a similar problem here with my 7.5 month old dd. I am still trying to figure out what to do but I had some thoughts about your situation.
When she wakes up after 30 minutes, is she happy or upset (clearly still tired)? If she seems happy, then maybe that is all she needs?
Also, you said you tried increasing her activity time by 15 minutes. For some babies, increasing by a whole 15 minutes is too much for them and it needs to be done more incrementally. Like an extra 5 minutes one day, the next 10 minutes, etc. etc. Maybe that extra 15 minutes is getting her overtired?
Is her 3:30 nap interfering with her going down for the night? I ask this as you said you wanted her to nap longer in the early afternoon and was wondering why you feel that nap must be longer.
I don't know if I have helped at all, but thought I would give it a shot! I feel your pain! Good luck and let us know how you get on.