Magento 2 help to solve current issues

We currently have a website in the last stages of development before going live and need some help to fix a couple of issues and help us go live, we have been let down by current developers as they seem to of disapeared off the face of earth after promising to complete the work this site has been migrated from magento 1.9 to 2.2

current issues we are having

Checkout - the complete checkout button doesnt seem to work until you refresh the page for some reason also having an issue with paypal getting that to work but i think thats a issue with magento 2.2.4

Im also trying to create some custom catergories and want to add some custom CSS script to certain catergories

Multi site setup - we have already created themes but for some reason we can't select the theme in the admin as the save button doesnt work when we choose a theme so just need some help finishing the multi site

i will be looking for someone who has knowledge of
Magento 2.2
magento themes
magento migration