Update:
It's now 105 days since my CP18 letter receipt, 117 days since I actually received the letter. Called and spoke to the IRS again, which confirmed that yes, my case was still "in transit" which is code for "in limbo". I did find out which office my case was being handled out of (Ogden, UT) and the examiner I spoke to did indeed confirm the refund was being held up due to the $7500 credit claimed. She also said that they (the IRS) was running out of time to reply and that my case hadn't even been assigned a case number yet. She wouldn't expand upon what she meant, but I'm thinking it has to do with the 120 day limit that was mentioned by someone else in this thread. As a result, she was kicking it upstairs to her boss and said that I should hear something from someone in 3-5 business days (I'm not holding my breath). She did also give me a fax number to the Ogden office, should I want to write a letter and supply documentation now that I've confirmed this IS the issue holding my refund up. It "might" help move things along she said. So that's the next step....sigh....
BTW, I also called the Taxpayer Advocate two weeks ago and after a long discussion they basically said they could not open a case due to extreme backlog and that my refund was not a matter of financial life or death. A bit more prodding and he finally admitted (as a public employee should have) that if I "insisted" a case be opened, he could open a case, but that it likely would be put at the bottom of the pile. I said fine, do it and got assigned to my local advocate office with instructions to call after 8/28 (today). So tomorrow I will nudge them as well.