Speed

Insufficient information.

With or without any wind? By 7 p.m., wind should have died down already.

No prevailing wind.

6 mph backwards due to a strong opposing wind on the sails.

I had to make a few assumptions like location, boat configuration and when this river excursion occurred.

The boat is not moving, they've come to a low bridge that doesn't open past 6pm , and must anchor for the night....

Get a fancy watch.

If you're sailing down a river riding a 5 mph current, you must be under power to maintain steerage, so we will assume you are under power and not just drifting....the maximum speed is usually determined to be 1.34 times the square root of the waterline length, so for example if your waterline length is 25' then it would be 5 times 1.34 = 6.7 knots or 7.71 mph, this then added to the 5 mph current speed would be 12.71 mph as a maximum...

...or 2.71 mph upstream.

Would a 10 mph katabatic wind fill the sail ? It's going 5 mph faster than the river. Would it matter if was just a raft. A bigger raft goes faster than a smaller one?

For maximum efficiency a sailboard is the way to go....

A few posts ago, your presumption was no prevailing wind.

A katabatic drain (wind) is cold air high in the mountains sinking quickly down river (in this case) in the evenings. All day heated air rising, after sunset cold air sinking . Thus the question about down river sailing, with no prevailing "wind".

May one assume that the relativistic effects can be ignored?

No, here is a possible answer ...10 mph down river drain, river flowing at 5 mph.

<...at 7pm...>

Why is the time of day of any relevance, please?

Katabatic wind (downward) ,evenings, anabatic (upward) .wind up until sunset

in the mountains, Analytic wind (upward warm air movement) all day, then at sunset, all that air cools and flows quickly downward katabatic wind. It is basically a temperature differential which drives the climate of the world. If there was no differential, there would be no wind. Cold pushes hot.

Who cares? Just enjoy the view, and sip a fine scotch. You're obviously not in a hurry.

Enjoying a down river cruise with my fishing rod, a beer, at 6 mph. Going with the flow.