[Q] Can't boot to recovery or download mode, only to CM topic

I made the mistake of flashing a CM nightly on my tablet, because now everything's broke. I had it set up with CWM and SlimLP alphas which were running fine, but now everything's bunk.

I've tried turning off the tablet, but it just starts back up again without warning, even when unplugged. I've tried every power/volume combination at boot, but it only boots to Android.

I hopped into an Android terminal session and typed

Code:

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su reboot download

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and

Code:

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su reboot recovery

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as well as

Code:

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su reboot bootloader

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which sends the tablet back to Android post-boot.

ADB recognizes it as an ADB device, but it will only show as offline, and revoking the UBS authorizations doesn't prompt them again later.

I've even tried Odin Mobile and UAMP, as well as ROM Manager, but they won't flash anything either. Rather, the things they flash don't stick after a reboot.

What do I do?