Hi,
1/ Do you have Javascript errors in your browser console ?
If not ; would it be possible to have a front-end access in order to see the page ?
2/ The best is to use a mass action and modify the product field "product_vendor_id".
3/ The email "order_status_notification" of HikaMarket uses some tags:
<!--{IF:acl.billingaddress}-->
<td>{VAR:BILLING_ADDRESS}</td>
<!--{ENDIF:acl.billingaddress}-->
So if the vendor does not have the "billing address" ACL ; the billing address won't be added in the email.
Otherwise, you can edit the email (html or preload) in order to remove the blocks or change the content of the acl variable.
4/ HikaMarket proposes the "vendor" characteristic which allow vendors to create variant for an existing product.
The default variant is still define in HikaShop and it will require some custom code in order to change dynamically the default variant (or change it in the database depending the variants prices)
Regards,