Who said anything about intermediate stops? I'm under the impression that the train is planned to run nonstop LA to SF, which frankly makes sense, as the service wouldn't be convenient for anyone traveling to or those cities. Those guys can take the Coast Starlight
Looking at Brightline, my suspicion is that they would
probably have (or want to have) 2-3 stops at each end. There's no reason to "throw away" LAX-SJC traffic (or SFO-BUR traffic) when the arrivals would still be around 0730-0800 (and the departures before 2300). That being said, I could easily see running non-stop for about 6-8 hours in the middle. Note that the Spirit of California did pick up/drop off a decent number of riders on each of the ending segments.
Runtime is a challenge here due to the possibility of the crew dying (departing at 2200 and arriving at 0830 leaves almost no margin for error once you account for report time), but the possibility might be there to contract with one of the commuter agencies to cover them (i.e. just arrange for an operating crew swap at SJC or SBA). Doing this, particularly on the north end, would be helped by that being "commuter territory"...so they might be able to fiddle with something.
I'll vouch for the Spirit of Queensland's seats - I rode on them back in 2016/17 and they were good. I think a second-generation version of that design could work
very well.
As to coach seats? I'd probably tag a coach on. Nobody
likes it, but I could see folks springing for it. Look at how many pax take the Silvers from the NEC to Florida in coach (it's a majority of the market). The Auto Train regularly sells six figures of coach tickets each year. Now, it's quite possible that in both cases if the equipment were available you could sell an all-sleeper train, but IIRC the per-car yield is actually pretty decent in coach (and can be pushed up somewhat if you're not squeamish about doing so). So a train with 4-5 sleepers, a cafe, and a coach seems reasonable.
[As to doing so with old equipment? I'm never going to say never. If they could equip 3-4 trainsets with them (probably cheaper than two sets of new stuff), that might do the trick, especially if they're willing to gut the cars and overhaul them. And heck, maybe VIA will decide to dump its remaining Chateaus...]
As to UP talking to them,
@railiner , if I had to guess UP will probably run just about anything for enough money.