1) What type of traffic does Tixati use IPv6 for when it is available? Does this include file sharing?

1b) UPnP is enabled on the router – does Tixati use UPnP to allow inbound traffic on specific ports for IPv6, or is this feature only used for IPv4?

1) Tixati uses IPv6 for peer and DHT connections, including Channels peers.
2) I don't know. I can't come up with a good reason why properly functioning IPv6 would require UPnP on the router. Strict firewall rules? Then it's not proper in my opinion or intended to be blocking.

You say IPv6 shouldn't require UPnP, but if the router has a firewall that blocks unsolicited incoming traffic — unless it's part of an existing outbound connection — are there other protocols or mechanisms (like PCP?) that Tixati use to allow inbound connections?