I'm sorry to hear about the confusion surrounding this matter.
Each request to place an account on the Multi Account is evaluated on a case by case basis but, if a seller uses multiple eBay accounts, we are generally willing to place up to 2 accounts on the Multi Account Plan is his/her primary account is subscribed to a plan that includes Unlimited listings.
Our overall intention is to allow such sellers to use up to 3 accounts (not just simultaneously) so, as you mentioned you were informed when contacting our Customer Support team with your latest request, we do not allow sellers to remove accounts from the Multi Account Plan for the purpose of placing new accounts on that plan.
If you are no longer selling through the eBay account assistance with Auctiva account B in your example, you could potentially link that account with a different eBay account by generating a new eBay token. This can be done by mousing over the “My Account” tab within the account, selecting the “eBay” option under “Settings”, clicking the “Get New Token” link and proceeding as directed.
However, if you have opened a fourth Auctiva account and you would like to post eBay listings through that account, you would need to select a suitable paid plan by mousing over the “My Account” tab within your account and selecting the “Pick a Plan” option.
We also will not assist you in circumventing these guidelines using the method you described above. I hope this clears things up.