You can use the following CSS code to fix the issue on mobile by going to Appearance > Customize > Additional CSS and pasting it there.
@media only screen and (max-width: 700px) {
.woocommerce ul.products.columns-3 li.product, .woocommerce-page ul.products.columns-3 li.product {
margin-right: 47px;
}
}
As for the banner image, because the height is so small it will look weird on smaller devices so it would be better to use a larger banner so that it can adjust better.