Agree with durhamlad - individual stocks only in a S&S LISA (or S&S ISA, for that matter).
And agree with GregandTre that a cash LISA makes the most sense. Even aside from all the tax questions, you probably wouldn't want to put money at risk when you're going to need it in just over a year. The risk/reward on gains vs losses in the market doesn't make sense.
I believe the government bonus will also be US taxable income - have never seen an argument otherwise, although I'd be interested if somebody has one. So that's the bigger chunk for the IRS, the interest will be miniscule at today's rates, but £1k of interest each year could result in you owing some cash to the IRS, depending on the rest of your tax return.