How to add single or multiple products to Magento cart from external page?

Landing page for Predator EKWB is a nice pre-sales page, but not built on Magento (WordPress is so much nicer for these things :)) How to connect external page ( to Magento?

How to add Order in Multiple Quantities of X support to Magento 1.9

Magento built-in Qty Increments is a nice set of functionality. It forces user to buy specific product in defined Quantities of X. The problem is that it can only be set globally. How to change this?