Two suggestions:
1) 711 EDC ratchet https://www.amazon.com/gp/product/B09MJYRQP4/ (this thing's a godsend)

2) Galvanox 1/4 bit extender: https://www.amazon.com/gp/product/B0BMNN8JB3/ Way cheaper than Leatherman's and has mostly good reviews. Seems to have some QA issues here and there, but seller seems pretty responsive to issues & does fast replacements. Would add a 6x2mm magnet if needed.

1. Yes, but you need to keep a tight grip on it
2. No, it fits in magnetic 1/4" drivers
3. 711L Ratchet
4. The smallest are the Skeletool(s) and Leatherman Curl

bit extender

You need the Leatherman bit driver to use bits with the ST300. It cannot use the bit extender on its own. The extra length from this adapter may make the extender wobbly and/or unnecessary.

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ratchet driver,

Also consider this ratchet driver. I know car stuff tends to require high torque applications, and using this style might work better.

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If you have need of the micro bit, this adapter can let you use one with the Super Tool.