Swiss Rail Pass — mandatory reservations on scenic lines?

for special trains with panoramic cars, etc

Yes, those ones only. If you look at the SBB schedule, trains requiring seat reservation will have a block "R" and a "Z" listed in the notes. Indicates "Seat Reservation Compulsary" and "Subject to Surcharge".

Thank you!

I am trying to plan a trip for five days, and if the weather is wet all days may end up riding the rails a lot. Good thing I adore trains ....

One more question: the Gotthard Panoramic express ends service on 20 October. Does this simply mean that the trains with panorama cars will end service, but that the non-panoramic trains will continue to run?

Regarding the Gotthard Panoramic Express, you can indeed travel the same route with regular trains. The route from Luzern is Luzern-Erstfeld-Bellinzona-Lugano; to find it on the Swiss Railways Journey Planner (SBB) you must specify a trip "via Goeschenen", as this will route you via the scenic Gotthard railway.
You could also take the ferry from Luzern to Flüelen, and just take the train from Flüelen to Erstfeld then as above. It takes about 90 minutes longer via boat.

Thank you.

I spent a while this afternoon playing on the SBB website. Other than it thinking I have a half-fare card when quoting rail fares it seems straightforward.

I do not know if I should be concerned if a train shows heavy ridership (three little people). Will I be able to get a 2nd class seat?

Then I got the crazy idea: “how many of the “classic scenic trips” could I link together in four days?” Which was an interesting diversion during a teleconference that I did not need to participate in.... ;-)

I assume from your post that you're travelling alone: if so, I wouldn't worry about seats. Especially if you start from terminal stations like Luzern or Zürich HB.
As for linking journeys together: it would be absolutely crazy (11-12 hours) but still technically feasible to leave Zürich or Luzern at dawn, ride the Gotthard railway to Goeschenen and Andermatt, then the Glacier Express route to Disentis and St Moritz, then the Albula railway to Chur and back to your starting point in a single day.

"Other than it thinking I have a half-fare card when quoting rail fares it seems straightforward. "

Yes, the default is for prices with a half-fare card. That's because almost all Swiss people have one (they are eligible for a different one than tourists get, and theirs is an even better deal).

Tourist Half Fare Card: CHF 120 for 1 month
Residents Half Fare Card: CHF 185 for 1 year
The residents Half Fare Card renews automatically unless you cancel it, you get sent a bill every year. Technically you don't need to be a resident, just an address they can send the bill to.

And the only trains in the whole of Switzerland which need reservations are the 4 (?) extra tourist trains. The regular hourly trains on the same route do not need reservations.

"Three red men" train full - I would say this means train > ~¾ full. Trains don't load evenly. Depending on where entrances to stations are, some parts of the train may be fuller than others. "3 red men" may mean standing in some parts of the train, and still seats in others.