Early withdrawal tax implications from megabackdoor?

I’m not a citizen of USA and may/may not move back to my home country after sometime. So traditional order of investment vehicles may not be the best option for me. I’m maxing out pretax 401K, not eligible for roth IRA and half assing the HSA.

My employer provides quite a comprehensive retirement plan with matches and mega-backdoor (after-tax 401k -> roth 401k conversation). If I have to withdraw early, Is it better to put money on megabackdoor or normal brokerage?

1) IIUC, for early withdrawal I would need to pay 10% penalty on the gain vs 15%(?) long term gain on normal investments?

2) Is the 10% penalty also applicable to megabackdoor contributions?

3) Can aliens withdraw retirement funds spreading over a few years to lower taxes?