Why not just call those things infuriating, bad, counterproductive, ruinous, negligent, futile, aggravating, or shitty?
"Retarded" originated as a medical euphemism for what is now sometimes called "developmental delay": some kid didn't learn to walk or talk or read or behave himself by the usual age. As a verb, "to retard" is "to slow down"; it's cognate to "tardy" meaning "late for class".
The original sense of "retarded" was about people with disabilities.
It was only used as a slang insult among schoolchildren because their teachers were using it as a medically descriptive term: "Be patient with Kevin; he's retarded."
The point of calling your friend Billy "a retard" as an insult was that you were comparing him to Kevin. The point of calling the school rules "retarded" was that you were comparing them to Kevin.
The whole reason that you think "retarded" can mean "infuriatingly stupid" is that Kevin flails and babbles because of his disability, and you're used to being infuriated by him instead of being patient with him like the teacher told you to.
You getting to use "retard" to mean something other than a disability insult is not "reclaiming" a slur. That's not what "reclaiming" means.
Reclaiming a slur is when the gay activist group calls itself Queer Nation, not when a straight person decides they want to say "queer" as a generic insult. It's when they say "Sure, you go ahead and call me 'queer' — because I'm going to use that as a good thing."
Reclaiming a slur is when we stopped saying "No, we're not nerds, stop calling us nerds, you mean jocks!" and started saying "Yes, we are nerds, nerd culture is awesome, you wish you were us."
Inflammable should probably be reclaimed for public safety reasons.