I will try and keep this as short as possible.

I have a G35 with a VQ35 DE engine. I had an APS TT kit fitted along with an engine build. (cosworth pistons, H rods, bearings etc etc). This all worked fine and the engine used very little oil. I then had an accident where i was 'T' boned and the car was a total loss (no visible damage to engine bay area).
I kept the built engine and when i got my replacement car i fitted this engine into it.
Pretty quickly i noticed i was using more oil than the previous car even though it was a straight change over without any extra works to the engine. I then noticed a puff of smoke when starting the car up in the mornings followed for about 3-5 minutes of slight smoking from exhaust but none after that while driving even when coming off the gas at higher speeds.
My mechanic has removed the turbos to disscount them causing the problem and believes the problem is my valves, either the stem seals or the guides. He says its not the rings because my car doesnt smoke when coming off the gas at speed.
So i have two questions;
Does this sound like a valve problem to you guys ?

Also is it possible to change the seals or work on the valves without removing the engine.

Sorry its a bit long but thought it best to give the whole story
thanks