minimum connection at Schiphol UK -> AMS -> Germany

My Question: Is it likely that an EU-citizen will be able to clear Schengen immigration and change piers in under 40 minutes?

Yes, probably better than 95%, but if you really do need to be at the destination that evening, you probably should add more padding.

I've done the minimum forty minute connection from non-Schengen to Schengen a few times (probably more than fifty in my time), and as far as I remember only one of those went badly and caused me an overnight stay in Amsterdam. I used to be based up in Leeds where the only air route on a network carrier was down to Amsterdam on an Embraer or a Fokker (until BA added a rotation to London).

The tight connection requires a bit of single-mindedness when changing planes but if you don't stop for a haircut between gates it seems to work out.

According to this TripAdvisor article from 2016, KLM Cityhopper use a remote pier, with a bus transfer.

Yes, informally known as the Fokker Farm. It's quite an efficient operation though, usually the bus is already waiting when the plane pulls into the space. You are decanted behind gate D6 which is close to the Schengen passport control for KLM departures.

The situation is much improved since they moved to central security; UK-originating pax don't need to be rescreened. Passport control for EU citizens is easy and fast, especially if the machines are on. Non-EU can take more time.

When will a flight be listed as "short connection" at Amsterdam Schiphol suggests that there is a special "short-connection" queue than I can use.

There was, at the entrance to Schengen security, but I have a feeling they dropped it since the only congestion now is passport control.

however, the only later flight to my destination is with EuroWings (Lufthansa, who are on a different alliance).

KLM will just rebook you automatically onto the next KLM flight and send you an email about it. I can't imagine them endorsing your ticket for use on a competitor these days unless you are very important to them and also capable of being very irritating. You'll get a hotel and meal voucher if you can find the right desk.