There will be a strong chance that there will be snow on the Arete in late April, and pretty much guaranteed on the Ben summit. As you say a lot will depend on the prevailing weather conditions. Best to take the Ice Axe and Crampons with you to FW and decide on the day.
In late September there was an half hour whiteout on top of the Ben.
Timewise my route from the centre of FW, Lochan Meall an t-Suidhe, CIC hut, CMD, Arete, Ben sumit, return to centre of FW by Tourist route took 12 hours. Your route is much shorter and should take much less time.
Enjoy
Robin.