As I understand it, the active radar return does not carry any altitude information... only direction and distance. Therefore, if the transponder signal coming from "bogey" ghost suddenly shows the aircraft at 1500 when it actually is at 6000, that information had to come from the transponder... which wouldn't send inaccurate info. Can the "ghost" transponder signal garble the information to indicate the wrong altitude?
That depends on the radar. Some provide three dimensional data. But in this case, the PASSUR system is not provided anything but transponder data.
"Therefore, if the transponder signal coming from "bogey" ghost suddenly shows the aircraft at 1500 when it actually is at 6000, that information had to come from the transponder"
No. Read this again from the PASSUR site "Aircraft tracking radar, and the software that supports it, while highly reliable, is also complex. Sometimes circumstances can interfere with the signal, causing temporary distortions."