Nile flow, about 7mph (c.10km/h), but with sail could do 15km/h, so about 1.5 hours sailing to Akhet drop off point.
Tacking may need 4-5 hours, and/or with rowing may vary depending on wind (or lack thereof) from 3 to 7 hours. Easy to leave in the morning from the Akhet drop off to get back to Tura. Merer does not need to reload for the turnaround, as he is not a vessel captain, and easily swapped to oversee the next/other transport.
Aside from river-route return to Tura, land speed by camel could also give plenty of time to return to Tura for the next task of oversight.
That some of these thoughts do not occur to you whatsoever, you either 'want' the Merer diary to be in error to support the greater fantasy of the GP having nothing to do with Khufu, or you are just too plain bamboozled by the 'how is this possible?!?' question for your brain to shut down. C'mon, Philip. Turn them little wheels.