How much time needed to get from train at Interlocken Ost to the Pier?

We will be on a train scheduled to arrive at Interlaken Ost at 10:48. We will have luggage and need to store it. We are scheduled for the Brienz-Rothorn train at 12:58 PM and hoped to take the ferry there. However, it appears that only one ferry will get us there in time. It leaves at 11:07 and arrives at 12:20. 19 minutes feels like it is really cutting it close to find the luggage storage and get to the pier, especially considering that we will have no idea where we are going.

So, instead:

Is it possible to just buy the tickets in the moment rather than in advance? I was also confused by the site because we have a half fare pass plus family pass, and I can't even reserve the free family pass online, so I would only be buying one ticket online anyway. (It seems like for the second ticket, you just show the pass when you get there?). If we don't make it, we can just take a taxi (or train, if there is one, to Brienz).

As we would like to take the cruise, the other more laid back option is to take the 17:40 cruise back from Brienz to Interlaken Ost. That means spending an hour in Brienz, which is probably a lovely thing to do!

Do we need to prebook either of these cruises?