[Thomasb134]

1. Can't help on this one.

2. The php manual states that mktime()'s $is_dst param is depreciated, not the entire function. So look for mktime() useage that has 7 parameters (ignore those with 6 or less). The 7th param (when used) is the depreciated $is_dst. As a cheap workaround perhaps you could try removing the offending extra param and see what happens.

3. The weird names in the templatesC directory are the compiled templates. Have you tried admin->Maintenance: Clear templates/X-Cart cache?

Or maybe temporarily disable Settings->General Settings: Do not check if templates are changed (Smarty compile_check), and see if any of the related warnings are suppressed. Not sure if you have this feature in your version, so don't panic if it's missing.