Your hypotheses are all correct, as long as a few little assumptions I’m making are also correct (!). Since your parents live in California, I’m guessing that you’re either a U.S. citizen or resident, so reporting worldwide income here is applicable. And since you concluded your work in the U.K. in December, I’m assuming you then returned to Washington D.C., where you’re now staying, so that the Foreign Earned Income Exclusion (discussed here) would be irrelevant, as it requires a longer stay. (The part I can’t make any assumptions about, or even comment on, is the filing obligation in the U.K., since that’s a completely different tax world!)
I do have some additional thoughts which might be helpful, mostly on your state scenario. First, if you use your parents’ California address on your U.S. 1040, the state of California may contact you to ensure that your income isn’t taxable there (which shouldn’t be a huge deal—just something to be aware of).
Also, since you left their state for just a few months and returned (see assumption above), D.C. will consider you to have been a resident there all along (states are aggressive that way about temporary foreign income), and tax you on the stipend from the U.K. in addition to the income which you actually earned in their state. But this takes place somewhat automatically in the TurboTax program, as it starts the state module with your federal AGI (and as long as you don’t enter the stipend on the “subtractions from income” page you’ll eventually come across, you’ll avoid headaches later).
As long as you’re a U.S. citizen and paid tax in the U.K., you would likely be eligible for the “Foreign Tax Credit,” which I believe you’re already on top of. You’ll see this as you go through the interface (but here’s a TurboTax article on the process), and we’re here to help if needed (it can be a little tricky!).
I hope this helps, @sfowjr66!
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