is it possible to belong to the kynareth temple and the mages guild

Yes. What you cannot do is join two different temples or two different knightly orders.

joining kynareth and the mages guild is kind of a wash, since they both offer spellmaking services, and kynareth & the mages guild doesn't offer potions. the same can be said of the mages guild and julianos. i believe those temple are ment for the players that do not wish to develop spellcasting schools to access those services.

Powergaming isn't everything

no, but it's worth noting that the temple of kynareth & julianos do not offer potion services, and thus the player would need to be a murderer to have access to the potion maker. joining the mages guild in conjunction of those particular temples just means the player has redundant services. julianos and the mages guild openly hate each other, which causes promotion issues, i think kynareth stays neutral (which is odd as the mages guild should hate anyone offering same services).

those particular temples are good if you don't join the mages guild and want access to either spells & spellmaking, or magic items and item maker.

Yes, but xkotekull is determined not to drown anymore by any means necessary, so Kynareth is a good choice