Hi,
1. You could do a plugin, but even simpler, you could have them as view overrides.
Simply copy these files in the folder templates/YOUR_TEMPLATE/html/com_hikashop/product/ and they will be used instead of the default ones and they won't be overwritten during updates.
That's the basics of view overrides in Joomla:
docs.joomla.org/How_to_override_the_outp...rom_the_Joomla!_core
2. It's not impossible but much more complex. That's because the whole checkout is inside a form of HikaShop. And the easysocial module login has its own form. If you try to put a form into a form, the form inside won't work and it's even not valid HTML.
So it would require hacking in the checkout system of HikaShop to remove the form of the checkout on that step to have only the module form and then have it redirect back to the checkout once logged in. It's possible but will require a skilled developer knowing how Joomla works at least a bit. I'm afraid that if you're developer doesn't even know about view overrides, it will be difficult for him to do that.