I managed to get the custom form working on the below page. However, it just shows consent for Analytics and I am not sure where that is coming from. Also what is not clear at this stage is how tags (e.g Google Analytics) would be enabled/disabled based on the consent provided.
Consent categories displayed are based on the categories you chose for your tags in Piwik PRO Tag Manager. There’s by default one tag enabled (Piwik PRO Analytics) which has the Analytics category selected by default. Hope this answers also the 2nd question - simply add a new tag with GA code and assign a proper category.
Regarding your second question - did you add any custom code on that page? If the consent form is enabled and custom consent form setting isn’t enabled, the form should simply pop up.