What I meant to say is that if you use a non-PD source, the current limit will be set to 500mA, which may be enough to slowly charge when idle, but will likely make it slowly discharge while in use. However, even for non-PD sources you can raise that limit, but it needs to be done manually.
Yes, that’s true. Generally, charging speed is limited by two factors - charge level and input current limit (which is negotiated via PD and defaults to 500mA for non-PD sources). Maximum charging current is around 1.5A. If input current limit is high enough to provide enough current to power the phone and charge at maximum current, it means you’ll be able to charge at full speed.
Not a GUI, but all these things (and more) can be checked via /sys/class/power_supply.