I would use a non animated slide panel in the popover. Each panel needs an object name. One displays your existing log entries and the other has a single row portal and go to portal row last for entering your new records.
Tthe trigger is on the pop over itself and not the button.
I would pass a parameter of "new" with the add button that will drive you to the correct slide panel via script.
scracth that. Since it is a dedicaTed popover for adding a log you don't need the slider or the parameter. The only thing you need is a trigger for a script to got to the last portal row for entry. Assumes you have record creation via relationship enabled.
isn't "Go to Portal Row [Last]" going to close the popover immediately though?
To disable script triggers I set a global variable, $$TriggersOff to True. I put this code at the beginning of every script performed by a script trigger:
if [$$TriggersOff ]
Exit Script [. ]
To add related records from the context of the pop over, I recommend using a method known as MagicKey. You can research this term to learn the details. It's advantage here is that the popover can remain open while the script adds the related record.
You can place global fields to the popover for entering data that can be used to create/edit the new related record. An edit button in the portal row can transfer data to the globals and the portal record's ID to the the MagicKey match field. This way, the same script that creates a new related record updates the record selected for editing instead.