1. I do not live in Australia so this question is a bit difficult for me to answer, generally though missing person reports on adults take quite a long time to even begin to investigate.
2. It makes a moderate amount of noise, but you can mitigate it. You may potentially vomit at some point, and you will likely seize. If you want to avoid loud noises, I would suggest laying yourself on the ground in the recovery position (on your side, face pointed towards the ground to avoid aspiration) to avoid the sound of you falling out of a chair or bed if the seizures are severe.
3 and 4. I do not have much experience with actually testing SN samples, but the instructions on the test should be pretty clear. If you got your stuff from DMC, it is very likely to be pure, so I wouldn't concern yourself too much if you cannot figure out the testing.
5. Your antiemetic isn't the top choice, but it does have significant D2 activity which is the most important part of SN antiemetics. The drug strength is still likely intact for the most part. I'm not familiar with the dosing for the drug, but make sure to take enough to account for potential purity losses due to decomposition of the active ingredient.
6. Any kind of long-acting antacid combined with a short acting antacid on the day of will work. Sodium Bicarbonate for the short-acting, and I'm not sure what long-acting medications exist in Australia, but your chemist would probably know.
7. In terms of the sedative medications, I won't recommend a dosage but you should probably take a little more than what you would normally take to feel a strong sedative effect. In terms of the painkillers, take a minimal amount to knock out any headache or discomfort you might feel during the process. Don't bother with the migrane drugs. Also don't bother with the seroquel, it has the same underlying mechanism of action as the antiemetic plus a bunch of other random effects that could interfere with the process.
8. Discontinue all medications beyond the ones needed for the process.
9. The best way to kill SI is the aforementioned sedative medications.
10. I have no advice for frame of mind. It is your decision to make, if you feel it is right than you will make it.
11. Fear of the unknown is natural. If you're feeling discomfort day-to-day, you can always take some of your sedative medication, as you seem to have enough.
12. You can continue your anticonvulsants to potentially offset seizure risk, the sedatives will also counter it. The seizures with SN typically occur after the onset of unconsciousness, as the brain is oxygen starved and cells are dying. It shouldn't be painful for that reason.
13. Use the PPH Protocol for SN, it is somewhere in the suicide megathreads sticky. Replace the meto with the proper dosages of your chosen antiemetic, and take the sedatives and painkillers in the dosages you desire the day of an hour or two before SN ingestion.
14. I didn't notice anything you are overlooking
15. Unsuccessful attempts are rare unless you are discovered. If you are stopped or it does not succeed, you are extremely unlikely to end up with neurological damage.
16. Don't drink alcohol. It slows gastric emptying and increases your vomiting risk.
17. No, making the drink isn't difficult. Follow the dosing guide on the sticky, and dissolve it in around 50ml of water. If you aren't exact on the water, it really doesn't matter.
18. Unconsciousness times vary wildly, typically it is within 30 minutes to 2 hours. Not as familiar with this part, but there's a lot of resources here.
Let me know if I can answer anything else. I hope everything works out for you, one way or another.