By creating Company 'X' are you not diluting the brand equity of both firms , in which case you can always go for 50/50 partnership and reconstitute the board of directors accordingly. If the agreement does not work out you can each opt out as per the agreement exit clause.
OTOH In the event that from inception , company X is a re-branded re-packaged version of A , I don't think it's a fair deal for you in any case , Coz if they pull out at anytime you have to start from scratch to build your online retail brand again.
The online platform as u put it is meant to be an external driver , while the real business model is organic .
Also it's foolhardy to expect 4x and 5x returns from the word GO, I don't understand the merit of arriving at business decisions based on speculative details in your conversation with Company B.
In order to engender trust between both parties I suggest a succinct study into underlying dependencies in integration , marketing strategy and assessing sales pipeline, (the expenses to be borne by Company B), which would enable you to assess the potential risks involved.
In the enterprise consulting space that i was a part of , when in discussion with a customer like company B, we always look at business blueprinting as a separate T&M exercise to be borne by the vendor .
Once the client is made aware of the kind of efforts , the cost of integrating an online system , they calculate the potential ROI, and then decide either to go ahead for such an integration with company A product offering .
All the best .
U can share specific details by PM.