Ted here from Shopify, thanks for posting your query!
The "payments on hold" message you are seeing is in regards to your Shopify Payments account. It may take a few business days for our Shopify Payments team to approve the documentation your client has submitted but if they need any more information regarding the review, please have them contact our support for more info.
In regards to the Bogus Gateway, you are more than welcome to use this while the account is under review. You can enable this feature as shown in this guide. Deactivating Shopify Payments to enable the Bogus Gateway will not affect the clients account review.
If there is anything else I can help you with, please let me know, I'm happy to help!