Just to be sure, and for other readers: that's not Rail Plus Australia (a ticket retailer), it's a card for Swiss residents on top of their GA, to get a discount on international tickets.
Now, back to your questions:
"Does this apply to Swiss Half-fare Card and Swiss Travel Pass Holders as well?"
No, it does not. It is important to realize that while the Swiss Half Fare Card and the Half-Fare travelcard are similar, they are not the same thing: www.myswissalps.com/sw isshalffarecard/detail s. The former is for tourists and does not offer discounts on international tickets, the latter is for Swiss residents and does offer those discounts. The SBB website is primarily geared to Swiss residents, and somewhat to residents of surrounding countries. For tourists, their site of course offers a wealth of information and an excellent timetable, but when it comes to tickets one needs to be careful. Domestic tickets are fine though, and picking Half-Fare travelcard there is ok (i.e. I'm not aware of exceptions).
Likewise, the Swiss Travel Pass (for tourists) and GA (for residents) are identical but not the same thing. The above applies here as well. Of course with a Swiss Travel Pass no tickets are needed for train trips in Switzerland so there's no need to even start the buying process, other than checking what a trip would have cost normally.
Regarding your hypothetical trip from Zurich to Paris:
"My question is which of these options equates to the "free" Eurail Select reduction?"
Well, none, actually :-) This is another example of the SBB site being mostly geared to Swiss residents. Other sites are geared to tourists (like the ones listed here). However, in this case the solution is simple: if you have a Eurail Select Pass for both Switzerland and France, the trip is covered. You do not need to claim a reduction for a trip that is 100% free anyway. This is similar to trips within Switzerland with either a Swiss Travel Pass or any Eurail pass: if the route is covered you do not need a ticket, not a free one either. The only thing you may need is a seat reservation if the train of your choice requires that. Reservations can be purchased wherever you like: at a station (although that is a bit late), from SBB, any of the retailers listed on our website (e.g. on the page linked above, or here), or even at https://www.eurail.com /en/reservations/eurai lcom-reservation-service (along with the pass or separately). All of this applies to Interrail passes as well by the way. A reservation is a reservation, regardless of which pass or ticket you combine it with. There are some exceptions in France, but let's not go there :-)
Another situation is if you have a pass covering the country you depart from, but not the one you are going to (or vice versa). In that case you can buy a ticket for the leg that is not covered only, as explained under "Traveling beyond the area covered by the pass" on www.myswissalps.com/sw isstravelpass/howtouse. Alternatively, you can buy a ticket for the whole journey, while claiming a reduction for the pass you own. Not all websites offer that option, but several sites that we link to throughout MySwissAlps do. They actually offer the option to select the Swiss Travel Pass (not GA), Swiss Half Fare Card (not Half-Fare travelcard), or Eurail. The advantage of that is that the seat reservation (if applicable) is included for the entire route, and not just from the border. I wish more websites would offer that, but I'm sure this will improve over time.
Does this answer your questions? The world of train tickets & passes can be quite challenging! And it keeps changing so of course we'll keep an eye on things.