Solve Issues on Magento 1.9

Only apply to this project if you can start right away, because I need it done in 48 hours max! This is very important, my store is losing sales until the problems are solved! I need some problems on my Magento Store to be solved. They are: - Add to cart button doesn't work properly. Sometimes it adds the product, other times it does'nt add, and leads my customers to an empty cart. - Onepage checkout is not working properly too, only guest checkout is working. Trying to login leads to their old cart (missing current guest cart), or to a 404 error page. Register during checkout also doesn't work, ending up in a 404 error, or an empty cart. What's strange is that the store works differently in mobile and desktop. I already tried backing up 2 months, tried default theme, and other things. My Magento's version is CE 1.9.3.4 and the website is [login to view URL], it's in brazilian portuguese.