Hi,
As I said, you can already do it with the HTML add to cart button with a view override.
The code is:
<a class="hikabtn" href="index.php?option=com_hikashop&ctrl=product&task=updatecart&quantity=1&cid=xxx&Itemid=yyy">Add to cart</a>
where xxx is the id of the product and yyy the id of a HikaShop menu item to use for the process.
If it's a static button, you can put that HTML code anywhere.
If you want something dynamic, on the product, you can edit one of the view overrides of the product page (for example show_default) and add that code and replace xxx by:
<?php echo $this->element->product_id; ?>
Again, this will ignore variants, options, custom item fields and the notification mechanism. It will add the product to the cart and redirect to the checkout.