Enterprise licensed products are automatically registered to the purchaser via the CLP/TLP storefront (now replaced by VIP/ETLA) and appear on the Volume Licensing Website, so if Customer Care say the Acrobat XI Pro license never existed in your account then it's not legitimate. I'm not saying why that is, but of course Adobe won't help you install a product you can't prove you legally own. Hence you need to talk to Customer Care - not about the technical issues with Acrobat DC, but about where you bought the Acrobat XI license and why it's not showing in the database. As I'm sure you appreciate, software volume keys are often abused by hackers and illegal resellers (e.g. by giving the same key to several customers), so Customer Care is rightly suspicious when someone complains their serial won't validate.
If you need a working copy of Acrobat XI Pro urgently to complete your tax return, then you can simply install the trial version from ProDesignTools - it will work for 30 days. Run the Acrobat Cleaner Tool first to remove all traces of the DC installation.