@gwalder , concerning your question :
" 2. in 2020 I had some foreign income from a job, that was taxed at source. can I report that on turbotax and benefit from the UK/US tax treaties to avoid double taxation? "
Generally , you should be able to avail of either (a) a deduction for the taxes paid to another taxing authority within the SALT limitations OR (b) a tax credit . Note that the tax credit while recognized in total , the actual allowable for the year is based on a ratio of foreign income world income ( the taxes thereon) -- the unallowed portion can be carried back one year ( if there was foreign income ) or forward for 10 years ( again need foreign income ). You said the income was for 2020 ( earned during calendar 2020 or paid during 2020 ? ). Were you in the USA while earning this income i.e. was the work done in the UK or what ?. When did you enter the USA and with what visa -- trying to determine exactly when you became a resident for tax purposes ( or Green Card ).
Need answers to be sure that I am in the right ball park
