Kyogre has more bulk than Palkia and specializes as a Water-type more rather than taking mixed options; I think it's superior to what it has to counter. Palkia has boons over it for sure but for strict Water-type usage I think Kyogre edges it out.
Walrein / Lapras are honestly so similar that I think Ice Beam Lapras just has a slight boon over it. Pure preference - if you can dodge both Blizzard and Ice Beam well, then it's not like either outdos the other. If you can't dodge Blizzard consistently then Walrein will pack more of a punch. Offensively, 2 bar moves (Ice Beam) offer more mileage by saving energy unlike 1 bar moves... Stuff like that.
Why bother with Mamoswine? For one it's infinitely more accessible than Kyurem is (especially when you can catch level 35 ones), and two, Gen 5 is ages away. Granted, this second argument can be made for the Dragon slayer Kyogre set and Mamoswine, but I consider being able to use a Level 35 Ice-type off the bat much more convenient than powering up a level 20 Legendary, esp when Pilo itself is actually not a horrible option of an Ice-type while Mamoswine gets here.
I don't believe the "power up only what the strongest is" personally. You don't have to care about power-up costs because the gens are so far away you will make up what you used in stardust, and more. Example: All the time you spent without a Water-type Kyogre because you wanted to wait for Palkia is raids you have missed, damage bonus you have lost, raid bosses that have fled, etc. I will always max out what is relevant this gen because waiting for something like Kartana to come out in Gen 7 just to say "oh it's the best Grass-type I'm gonna wait for it" is genuinely limiting. Maybe for more constrained playstyles that don't get a lot of dust, the argument makes more sense.