One question per thread, please.
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I'm not sure what you mean. What does x64 have to do with copy and paste? Opus is a native 64-bit app so I imagine it'll behave the same as Explorer, but the clipboard is the same whether you are 32-bit or 64-bit (as far as I am aware) so I'm not really sure what we're talking about here.
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Decompressing RAR currently uses the 32-bit version of unrar.dll, even in 64-bit Opus. In the not-too-distant future, Opus will use the 64-bit unrar code. (Apparently it's slightly faster, but either version will decompress things the same so that should be the only difference for now.)
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Not as far as I know (you could send a feature request) but you can add a slider to your toolbar that lets you adjust thumbnail side. (And it'll be hidden when not using thumbnails, so it doesn't clutter up the toolbar.) An example toolbar with this setup is here.
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What command does your hotkey run?