With script editor, currently anyway, AFAIK...
- you can alter the quantities of products in the cart, but you cannot add any new ones.
- you can change the price of any line item
Been there. Done that. I guess that just parrots what you just said. If you want though, you could always triple the price of a product too, so, it is not limited to discounted or lowering prices.
Interesting. No one has ever asked me to charge more for a product in a Plus script, so when I said you could triple it, I guess I was wrong. Oh well. This is a minor topic of interest anyway. I do not think the OP had any intention of ever using Plus.
You would never want to use Shopify Scripts as a product configurator. They are not made for that. You do not have nearly the support you'd need with the limits in scripting. They are meant to be mean, lean, fast little routines, that cleverly do a lot of the stuff those BAD APPS used to have to be used to do.