>> so I looked at a day trip from Wengen, say 8.24 am departure.
Have you seen the Grindelwald Bus page with information and a downloadable brochure with a timetable on how to do what they call the "Glacial Waters Round Trip"? Check again after April when they publish their new summer timetables:
There is also a video on that page to whet your appetite! It's in German, but the scenery speaks for itself :-)
>> Shows a 2 minute connection (Grindelwald train station arrival is 9.42 and the bus 128 departs 9.44). Presumably possible as timetable shows it as a connection. Is it reasonable? Just in case it's not.... as the day excursion is a long one, so missing the bus means an hour's delay.
The bus interchange is not right outside the Grindelwald station, but a little bit up the road, and involves crossing a road. I agree with Lucas that 2 minutes is probably cutting it a bit fine in this case. Although if you were in the first carriage, which means you wouln't have far to walk up the platform, it might be doable. However, I would get an earlier train to Grindelwald.
>> Not fixed on this, can leave cruise for another day (maybe do Rothorn and cruise together another day), and wander Meiringen and Brienz instead, they both look nice.
Apart from going to the Frutal Tea Room in Meiringen to sample their famous meringue desserts, personally I wouldn't spend any additional time in Meiringen, unless you are a Sherlock Holmes fan. It's a pleasant town, but there are much more spectacular options nearby.
Same with Brienz: great for access to the Brienzer Rothorn train and the lake boats, and just enjoying the lake view, but the town itself pales in comparison to other attractions. There is a famous woodcarving shop there though. Quality is superb, but they are quite expensive.