If we dock at 8 am and tender into Flam will we have time to make the 10:45 Flåm train if we have online tickets?
I should think so, the dock is right near the train station.
It's about 200 m from the port to the railway station.
Flam is a tiny town, as mentioned above its about 200m walking from the dock
This webpage has a nice overhead pic of Flåm's dock and surrounding area, complete with a docked cruise ship. It'll give you a good perspective of distance and what to expect. https://www.visitflam.com/activities/the-flam-express/ (scroll down about halfway)