With all that you said the bottom line is HD cannot work when the line is diplexed ( You should already know that ) so if HD is not working then the job was not done right. You may just have to run that new line ( neatly ) and everything will be working fine and you would have done a good job with a happy customer.
This is CABL.com posting #273290. Tiny Link: cabl.co/mbjf4