Hi,
A As you already know, we've been quite busy with the huge work on HikaShop 3.0 since a year ago. So we didn't had time to move forward as much as we wanted on other projects. It's still something we want to do but you won't see anything before several months at least.
B 1.2. We can't help on that.
3. I've reviewed your code and the only changes I see are for the function :
onBeforeCartUpdate(&$cartClass,&$cart,$product_id,$quantity,$add,$type,$resetCartWhenUpdate,$force,&$do)
You would have to use instead:
onBeforeCartSave(&$element, &$do)
and then, instead of using directly $product_id, you would have to add a foreach $element->cart_products to do your processing on all the new products by comparing them with the ones already in the cart (or doing it on all of them).
The rest of the plugin should be the same. So with that in mind, any PHP developer should be able to do that.