Hello.
1) No, it's not ABSOLUTELY necessary, SN alone will make you CTB
2) Yes, benzodiazepine (not only oxazepam, any other benzo) will be VERY helpful, because it will:
a) diminish/eliminate your SI
b) detach you from reality
c) minimize your anxiety/panic
3) What kind of "over the counter" sleep meds do you mean? Because only BENZOS are helpful, but those ARE NOT OTC, they are prescription. OTC sleep med which I can think about is only Melatonin (and maybe in some countries Hydroxyzine) - but they are so weak, almost like placebo. Melatonin does not have proven research. In other words: DO NOT THINK ABOUT OTC SLEEP MEDS --> THEY ARE USELESS.
edit:
and 4) answering your last question: after 40minutes of ingestion of almost every benzo (even those which take the longest time to work) of a big dose (PPH recommends a huge dose iirc) --> you will be VERY sleepy. And I mean it - VERY. You may even fall asleep = black out, before you ingest SN. I suggest not waiting 40min, but a bit shorter, in range of 20-30mins. Please remember benzos have "delusion of sobriety" and you may feel you are perfectly fine, and just a bit sleepy (or not at all), where in reality you will be f**** like after 10 beers or 0,5L of vodka. People around you can easily tell, but benzos have this delusion of sobriety that I told you about. Trust me, I use benzos daily for 10 years almost. And I used like 30 different kinds of them (they are very similar though).