US disregarded entity opening branch in Japan, very confusing

As the forum repeatedly mentioned, SMLLC are not disregarded in Japan as they are in the US.
When a US based SMLLC small business owner opens a branch in Japan on BM visa, it can have profits on both US side and the Japan branch.

Here comes some awkward scenarios:

1. the owner never pays themself any salaray in US since it's not required nor recongized in the eyes of IRS. They will need to pay themself in Japan, it's netted out when reporting to IRS, but it can make the income on both sides differ quite a lot.

Is the Japan salary attributable to only Japan branch or it must be allocated to US as well?

1. When the owner takes a distribution in US, it is not a tax event in US. However, my understanding is that it counts as dividends in the eyes of Japan taxation.

If so, is it considered US sourced or Japan sourced? If japan sourced, at what %

1. The owner has been paying US self employment tax based on the US profit amount. When the owner gets a certificate of coverage, are they exempted from pension withholding based on the US Federal Schedule C (Self employement) profit or just the entire Japan salary?

Side question: Would it be beneficial to elect C-Corp for the SMLLC to align the separation of individual and entity? I think it could be less headache but not necessarily tax favorable?