Dillution is generally a terrible idea, particularly if it's for anything court ordered. They see that clear pee and know, and make you wait then pee again.

Not sure what freezing does but mild microwaving shouldn't affect sample. I used a fake pee called Synthetix5 to pass a 10-panel GCMS test for my job and it required microwaving to get it up to temperature, then hand warmer and body heat to keep it there. It also had a thermometer strip attached so you knew for sure at the time of the test it was in the right temperature range.

The biggest risk of using a non-fresh sample is that bacteria can grow over time and that's a dead giveaway you didn't provide a real sample. Freezing obviously prevents bacterial growth but if it froze too slowly, it might have grown bacteria before freezing and those will thaw and continue reproducing when you heat it up.

If you have time, buy QuickFix or another trusted synthetic urine and just be 100% sure you have the temp right before you put it in the cup. Be calm, act normal at the clinic and you should be fine. If you have no choice but to use your old sample, be sure it's fully thawed, heat it up to the right temperature without overheating, then use body heat and/or hand warmer to keep it correct temp on your way to the clinic. I can't stress enough how imprtant the temp is.

Also they usually eyeball it for color and foam so your old sample will probably be better in regards to foam. Some foam is great, so if you can make some while you pour, do so. Synthetic urine lacks this detail and that's usually what will cause it to fail.

Another option is to have a clean friend piss in a sturdy bag shortly before your test and keep it in an armpit or your crotch going to the clinic to keep it body temp. Don't use female pee, though. If she finds out she's pregant you'll def. fail.