You want party line purity and discipline in an Amicus Brief? This is just a pbulicity stunt. A couple of clerks will be assigned to read through it and see if there is anything of weight and merit in it that the Justices need to contemplate. Otherwise, it is irrelevant to the question before the SC and the number of signators is utterly irrelevant, and in fact might even be viewed in a negative light as trying to pressure the justices rather than persuade the justices with a strong convincing argument not raised by the parties.
Correct. This could be a precedent fraught with peril. This piece has some more detail, one of which is merely time constraints.
https://thehill.com/homenews/house/529906-house-republicans-who-didnt-sign-onto-texas-lawsuit/
Winner.
You are precisely correct. This should be given precisely zero significance