If the Supreme Court rules 9-0 against the state of Texas in their lawsuit, will Trump supporters finally accept Trump lost fair and square?

Nope.  they will keep whining.  the only possible thing that could shut them up would be for trump to concede that he lost fairly, but that would take grace and maturity, qualities trump doesn't possess. 

They are going to keep crying long after 2024 when the Republican Party refuses to accept Donald Trump as a candidate.

No.  Trump supporters are basically a death cult now.  There is nothing that can convince them to come back to reality.  The only thing to do is prepare for their violent backlash.

Texas want their votes recounted. Why is it a Supreme Court case?

no....................

Nope.  Will just insist Deep State, etc

Texas has a pretty good case that they broke election laws.  I'm not sure what happens with a remedy for it.  They are going to be reluctant to toss the votes from four states.  But they will stop it next time and we'll all know it wasn't legitimate.