Answering your questions:
call the conference from an internal device which isn’t forwarded through inbound. It keeps telling me that the pin I provide is invalid.
Most probably you are using a softphone for internal calls, this looks like a DTMF configuration issue. I need to know what kind of devices or softphone are you using.
An other thing I’ve noticed is. That the PIN in the conference show a red line below it. Is that supposed to be there? See attachment.
Yes, it supposed to be there, ’cause it represents the password/PIN strength.