I'd rather have the entire freeway corridor under a single number, even if it does create a wrong-way direction for a bit.
Also, if we really need to free up an extra number, perhaps we could just extend SR 410 over SR 512 to create a nice continuous corridor. 
I don't see why a single number is necessarily desirable. There are already numerous examples of this occurring elsewhere: both ends of the 405 (where it becomes 518 and 525); southern end of the 705 where it becomes Hwy 7; Hwy 99 where it becomes 599 in Tukwila; the 101 where it becomes Hwy 8 west of Olympia and
then also Hwy 12 in Elma; I-182 where it become Hwy 12 in the Tri-Cities...probably another example I can't think of.
There are certainly reasons why these decisions were made, yes, but the cardinal direction issue is a good enough reason to also not use a continuous number here either. How does it make any sense, going southeast on Valley Ave, that you need to go
south on 167 reach I-5? Or going southbound on I-5 and taking the ramp to
northbound 167 to reach Puyallup...huh? The situation kind of already exists at River Road and I-5, but (A) it's more east-west compared to the new 167 which would be more north-south, and (B) literally everyone calls that road "River Road", whereas the freeway would only have a number (aka, no fallback).
As for the 410/512/167 situation: interesting solution. Although given the importance of 512 as an east-west corridor, removing it and moving it elsewhere, even if it kept it somewhat within the north-south rules for those 500-series highways, it would be really confusing for residents who have become used to the 512 number. I'd rather end 167 at 410, and keep 410 going all the way to the 705 in Tacoma. Still mostly east-west! I think a large portion of traffic on 167 comes from 512 or 410 anyways, so the idea of 167 continuing even past those roads seems hard to justify.