Going to be perfectly honest here, I don’t know if you can have two separate systems running independently from each other, but exporting out through the same system?
I’d ring up Octopus Energy as a producer to make a general enquiry. Please let me know the answer here.
Commercial or managed buildings, installers can’t install unless the property owner is purchasing and keeping ownership.
Yes, you can do a smaller off-grid with battery set-up, ‘type of installation’. These getting popular with garage, garden offices, boats etc. https://www.powermyhome.uk/diy-solar-panels/pv-solar-panels.php
Most modern PV installs come with apps to display generation data. As long as your wishing to forgo that tech, then yes, you can keep your existing meter if you specify with the installer.
Hope this helps.