You are technically right about not needing the whole trailing arms, but good luck with both finding the spindles by themselves in usable condition and taking the old ones off.
Finding parts like that is getting progressively hard as our cars start to turn into oldtimers, and most of these super specific upgrade parts have already been grabbed about a decade ago. We're kinda late to the party in that regard.
The spindles are also secured to the trailing arms with extremely strong threadlocker, which oftentimes are pretty old and corroded on top, which makes removing them near impossible without drilling them out.
Stripped a bolt attempting to remove the disc brake ones, even with applying heat and using rust remover beforehand.
No chance.
So, yes - you technically don't need them, but realistically you will.
Except if you're fine with drilling all of the bolts out and dealing with that headache.
And the part about bleeding the brakes is also generally correct. Most cars have their brake systems set up like that so you can still come to a stop if a brake line fails. Otherwise if you combine FR with RR / FL with RL you will only have working brakes on one side of your car, which I don't have to explain why that would be very bad.