Reporting income from a small business abroad with a spouse who is a non-resident alien

Yes, you would report the business income on Schedule C.

Yes, if this business is 50/50 with you and your spouse, then you would only report half of the income and expenses on your return.

 You are correct about having to only pay Self-Employment taxes if the amount is $400 or more.  

 To enter your self-employment income select the following:

1. Federal
2. Income and Expenses
3. Show More next to Self-Employment
4. Start next to Self-Employment
5. Yes, to you have any self-employment income or expenses
6. Enter the type of self-employment work you do and you can select that you use your name for your business
7. Click through a few more screens and you will get to a screen that says "Your XX info"
8. Click Looks good
9. Then you will come to a screen that says Let's enter the income for your business.  It will give you the option of 1099-NEC or Other self-employed income including the 1099K , cash and checks.  Here is where you will enter the money you receive.
10. If you do not select one, it will not let you enter your income
11. Enter your self-employment income on the next screen
12. Continue and on the next screen click add expenses

* Please note navigation may differ slightly depending on which version of TurboTax you are using*