Dear,

I would say appear in Naval tests, no problem, it won't take much of your time.
Apart from Pak Navy, you can appear in PAF as well until age of 26 yrs in ADWC or Logistics etc.

And just go for it, as far as CSS prep is going you are already doing it...
So lets see what would be the results of Naval Logistics Branch.
Secondly, do let me know is it a permanent commision through ISSB or is it direct select through NHQ (Naval HeadQuarters)?

ISSB is interesting, you would love the experience I bet!