There are two methods that I am aware of for sending email from a website … one is called phpmail (or something similar - I am not overly familiar with this) and the other is using a SMTP relay. If your website is a WordPress based site, I am certain that there is a plug in for this.
Now, in my opinion using the SMTP relayed method is the best. The reason is that the mail from your website will be sent via your MiaB. How it works is this, the SMTP plugin logs in to MiaB as one of the users on the box, so the email is sent as though it originates from the box. Hopefully, you can see why I feel that this is the best method.
This depends on WHICH DNS records. If you have not changed things since I helped you the other day, DNS entries pertaining to simplydriver.co.uk are entered into DNS at Wix. DNS entries for apuppydogstail.co.uk and jmail.rocks are handled by MiaB at box.jmail.rocks, so any custom entries for those domains need to be entered in the admin area under System>Custom DNS. Keep in mind that all DNS entries related to email for those two domains are already created there. The only custom entries that you should need to make would be if there is a website hosted elsewhere.
Yes, but … the determination is not made by the fact that the MX is pointing to a separate box, but rather by the fact that there are issues when dealing with Wix that you encountered originally - namely the inability to change name servers because the website is a Wix hosted page. That is purely a Wix limitation.