You can't update the SendGrid account informtarion for Cloud, you can specify multiple drivers for email in on-Prem, this is a limitation of Tyk Cloud. We understand the issues with modifyingbthe From field and we are working on finding a better solution. And we do not have an email verification step for the portal Ata lol, though we may add one in future.
As Kos stated, the portals are only configurable using CSS in the hosted solution, you have full control in on-Prem.
Realise that you stated that it isn't an option for you, but that is the way it is: our cloud offers a certain degree of portal capability and customisation but nowhere near as much as on-Prem, which offers full control.