Welcome to Just Answer. I will do my best to fully answer your question(s).
Not up to par this morning on my metric to English conversions however 6.5cm isn't a large offset. If the piping is done in PVC there is a 45 degree offset collar that will move your toilet approximately 1.5-2 inches in the needed direction and not require it to be moved forward away from the wall at all. This fitting is ridged not flexible so not issues with draining will an issue in the future.
I'd given this some thought - this would be a 40mm offset connector and it might do the trick to at least look a little better - however as the connection into the toilet is a little taller than to the waste pipeline some of this angle is already going to be lost. I'm also a bit of a finickity customer and was really hoping to get the toilet centred in the short wall space. It might be correct that it's not going to be easily possible without breaking the wall, etc..
Not quite sure I follow you about "losing some of the angle." If the existing drain is 4" pipe in concrete floor with tile then a small amount of concrete will have to be chipped away on the side in the direction of preferred offset to center. Then cutting a portion of the 4" pipe out of the way then this 3" collar can be glued inside of the 4" pipe. I have done this successfully many times in concrete. This will allow the collar portion to sit just where it should be on top of the tile. a small area of tile on the opposite side will likely need to be replaced to cove old collar location. Not sure if the is enough of an offset for you but the alternative is breaking up a lot more concrete to move the entire pipe. IF there isan ability to access this piping underneath the floor then I HIGHLY recommend you demand he cut out the entire piping and move it to it centered location. He should have done that anyways and if there was a reason he could not, THEN he should have went over that with you beforehand.
Hi - yes - for sure - having these issues come up at the 11 and half hour is not great - I just discovered it couldn't be moved because the neighbours (terrace house) is next to it. Thanks for the advise. I 'll go through the option you propose with him and will make a call. Thanks so much!