You absolutely can alter/reduce an accent, albeit with potentially large effort if starting from non-native; worst case, a speech therapist should be able to help, but even without that you can adjust by paying attention. I mean, yes, you can/should mitigate issues in the meantime but it is doable.

You can 'just ditch an accent' if you can mimic someone else's accent.

Yes, but that can certainly be overdone.

That's how you end up intending to say "Than you very much" in French, but actually saying "Thanks, nice ass!"

Not that I know from personal experience or anything.

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