I really liked Ryan Maccombs' idea (see https://webwork.maa.org/moodle/mod/forum/discuss.php?d=4462#p12973 ) of formatting input boxes for limits of integration nicely using CSS.
However, once we started using MathQuill for live rendering of the answer, that approach stopped working, as the CSS which applied to the original "regular" input boxes did not carry over to the replacement MathQuill input boxes.
I tried various approaches to find a technique which works with both MathQuill on and off, and the solution which seems to work the best is to set the entire "line" using a series of "display: inline-block" DIV elements, and to handle the layout of the limits of integration using "inline-grid". A small amount of CSS code is still applied to the specific "regular" input boxes by their ids (setting font-size, height, padding, etc.) but the majority of the layout is handled by the CSS grid.
Attached is a sample PG file using this approach, and images of the rendered problem appear below.