Of course. What you cannot do is stitch together the two implied inertial frames without doing so carefully. Did you carry out the computation I suggested in post #44 yet?I can remain in motion for 20 years, accelerate for one second, and then revert to inertial motion. It's not one or the other, all the time.
If you use the inertial frame in which you were at rest, and then naively switch to the inertial frame in which you are at rest after the acceleration, you've just changed your definition of "at the same time". It doesn't affect your history, but it does affect your assessment of what time it is elsewhere - and you fail to account for this change in assessment at your peril.And the one second doesn't change the prior 20 years.
You are treating him as such, yes. And you are finding paradoxes. Perhaps you should consider not treating him as such and see if the paradoxes go away? As we've been telling you for the last hour or two?The spacetwin is treated as if his motion in inertial the whole time.
It's a mistake to describe someone as not inertial because he's not inertial?It is a serious mistake to say that is is not inertial because he had to accelerate to achieve his crusing speed.