Hi!
Ad 1) 8 mA
Ad 2) Well not yet. Actually the HW PWM in Armada 38x work like that you can either disable or enable it on a GPIO. All PWM-enabled GPIO pins have the same frequency and pulse width. And you can only turn off individual pins. The intended usage is to use is to dim LEDs, which explains why it has this type of limitations.
And kernel GPIO driver nor the Python library do not support it. Sorry. But you might try to hack it yourself into the library (https://gitlab.labs.nic.cz/turris/python-turris-gpio). It should not be that complex - in HW it is only about setting GPIO blinkg registers and configuring GPIO bling counter ON duration and OFF duration registers.
Ad 3) Well, we have the above mentioned Python library. If you need C library you might want to look at source/c_gpio_omnia.c in the sources, which can be basically taken and used as the backend to access GPIOs over /dev/kmem.