Hi Gareth,
Thanks for your post here.
Based on your description, I understand that you are using Small Business (P1 plan).
As for the first question, I would like you to double check if the IP address you added is correct by contacting the support of .biz’s DNS hosted provider.
Besides, it is recommended to create a CNAME record.
For your reference:
Update DNS records to keep your website with your current hosting provider
And, if it will not work, please contact the support of .biz’s DNS hosted provider to confirm if adding A/CNAME record is supported.
As for the second question, I would like to confirm if you want to change the DNS hosted provider from Office 365 to others. If so, about whether the scenario is available depends on that DNS hosted provider.
Thanks,
Cherry Wang