You've made a decent parsing of shouldn't and couldn't...but honestly that's looking at the letter and not the spirit. (i.e. if that's all you've got...it aint much)
Republicans said that it shouldnt be done because it wouldnt get the desired effect. It happened and while it didn't get an OMGSHOWKANDAAWWWWESNAP! response...the guy's in for life.
So yeah, the idea that it was the wrong course of action, given the desired effect is wrong.
Wrong. That which cannot be done is not the same as that which should not be done.
The CLAIM made was that Republicans had argued that such trials COULD NOT be done. That claim is untrue.
What Republicans DID urge (and still urge) is that it SHOULD NOT be done since
it's a dangerously stupid idea. And that's true. It is dangerous, it is stupid and it's completely unnecessary to take such risks.
The balance of your post is kind of on the gibberish side.
It is a dangerous and stupid idea to treat the al qaeda captured combatants as mere criminals. What they have done and what they are doing is not a criminal matter so much as it is a matter of war.
If we captured a Nazi spy in the USA during WWII, we wouldn't give him a "trial" for some crime here in our civilian courts. At BEST (from his perspective) we'd hold him until the end of hostilities without any trial. It's not a question of guilt or innocence, and therefore a "trial" is absolutely beside the point. Indeed, a captured enemy spy (usually one engaged in "fighting" without a uniform) would be subject to summary execution. We may not summarily execute al qaeda fucks, but we sure as hell have every legitimate right to detain them without any fucking "trials" until the hostilities end. If that's "never," then that's when they get released. Too bad. So sad. Eat shit, motherfuckers.
Somewhere along the way, we lost our way. Our Courts went a bit rogue on us. And our politicians got led around by the nose by a bunch of ACLU types who also simply do not get it.