Feasibility of a partially remote postdoc
Ask the PI or recruiting HR department
I think the main consideration will be what the PI thinks of the plan. Some PI's want to be able to chat face-to-face to post docs every day, whereas others are happier with a more light touch interaction. This can also vary for the same PI between different projects with different aims. This will be quickly resolved by asking the PI. If you have a job offer there's nothing to lose here.
There may be other issues such as tax, insurance and other employment law things. I'm not an expert but our university has raised issues about staff working in another country even though most staff haven't been to the university buildings at all since March last year. This will be quickly resolved by asking the recruiting HR department. They should be able to give a relatively quick yes/no depending on their own current working pressures.
In long form,

explain as you do here what your constraints and motivations are.
have a concrete plan of how the remote part of the work will go. For example, are you able to shift your working time to be available for meetings during US working hours.
be prepared for the answer to be no.

You seem to have a reasonable fall back plan so I see no harm in simply asking.
Edited to add: You should also be realistic about how travelling will work during the current on-going pandemic. How will quarantine affect your plans? What if you are not able to travel for a prolonged period of time.