Have you forgotten why a US military was necessary just 75 years ago to save the UK from destruction?
Like you said, the UK saved itself (for now), but "American help was instrumental later."
I get the point you're making, but American military involvement had almost nothing to do with the UK repelling German invasion specifically.
I don't understand, in your made-up hypothetical, is there an Eastern Front or not? Or, are you mis-using "Eastern Front" to refer to the war with Japan perhaps?
As soon as Germany lost the Battle of Britain and then failed to immediately take Russia, their fate was sealed. The war was lost, period.
The next question was - would Russia overrun all of Europe (including Western Europe)? Without US involvement, that may have actually happened. Thankfully, it did not.