Worked out the answer with input from non WP-oriented forum and edited above to show.
In short the variables were retrieved from WITHIN the js function using getElementByID
and prevented error on non-action form with event.preventDefault();
like so:
function glitch_player_display() {
user_input = document.getElementById("user_input").value ? document.getElementById("user_input").value : 2;
generated_var = document.getElementById("generated_var").value ? document.getElementById("generated_var").value : "Default_Var";
$(document).on('submit', '#form_id', function(event){
event.preventDefault();
});//etc...