"Others have told us that the oral test has false positives and if you eat or drink 2 hours before the test it can cloud the results."
"Others" are wrong. Either there are drugs in your system or there aren't but drinking a soda and eating a hamburger before a saliva swab doesn't change your drug test. The ONLY ingestion of a substance that can give a false positive on a UA (don't know about saliva) is eating a poppy seed bagel or muffin. Poppy seeds can create a false positive for heroin.
"We've also been told that the oral tests aren't usually admissible in court... is any of this true?"
Nope, not when it comes to probation. Bottom line is he is using, the PO is on to it and he needs rehab. If he voluntarily goes to rehab there is a small chance that the court will allow him to continue probation. If he waits until they violate him it is likely to be much worse.
"That's just my opinion, then again I might be wrong." Dennis Miller