Sparkpost does not need to be set as an MX record to work if you are only using Sparkpost to send mail.
You only need correct TXT DNS records for SPF and DKIM to be set for Sparkpost.
I would say:
- set your general mailboxes back up, setting MX records just to Zoho (or another standard provider).
- approve your email addresses.
- Use Sparkpost just for sending mail from Discourse.
Seek-out, re-read Sparkpost documentation - follow it. Whatever you are doing now seems way off track.