how can I convert slow motion video into normal speed without losing fps or audio sample rate?

This is sadly not possible. You could theoretically have a video with effectivly 500 FPS, but that won't play on any device.

That's why cameras generally record to a project frame rate (Normaly 24, 25 or 30 FPS) and just stretch out the video.

This is a contradiction.

If you want to change the frame rate speeding up you have to change the frame rate.

You could try to playback a video at 16,000 fps... Why would you need to playback a video to 16,000 fps if no human or living being could notice the difference to let's say 120 fps?

You also need a monster of a computer to playback that among other technical problems.

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But then when I need it slowed down, I could use software or something to view it in slow motion

Ok, so have two versions of it, one converted to normal speed and the original.