Nice work on not falling into the xanax trap- it's a fucking shitty ride.
1) By erratic dosage I presume you mean not taking the whole dose at once? Because of how diazepam metabolises it slowly accumulates in your system (as various metabolites) and often has a 36 hour+ halflife, so splitting your doses will just mean that you'll have a smoother comeon and a lower plateau (which is probably what you want at work heh). I'm glad the diazepam helped with your backpain, as I mentioned- it has quite strong muscle relaxant properties that really help people with particular sorts of pain.
2) There's nothing 'wrong' with stacking nitrazepam and diazepam, they are closely related and have more or less the same effects. Just look up the equvilancy here =
http://www.benzo.org.uk/bzequiv.htm
and work out how much you're taking in terms of diazepam- it's not holy writ, but it's really useful in gauging 'how much' you've taken. Nitrazepam and Diazepam are about as potent per weight (10mg nitraz = 10mg diaz), so that makes things simple.
3) Taking between 2-8mg of klonopin (clonazepam) is the eqivilant of taking 40-160mg of diazepam and is a fairly long acting benzo- it's no wonder you blacked out from that dose, particularly if you didn't have a tolerance, 40-120mg of diazepam is a monster dose! Particularly for a beginner! If you want to take it 'recreationally', I'd reccomend starting with 15-20mg (maybe split dose, so you can gauge how trashed you are- try 2.5 5mg pills, wait 45-60 mins and if you want to take another 1.5 pills- so 20mg, staggered over 2 doses)- you will suffer some memory loss (just like with booze), but you'd probably need to take 40-60mg to suffer proper amnesia/potentially go apeshit and be made fun of when you're together enough to feel shame. Also, although a lot of people do it, I'd reccomend you don't drink on benzos- just take a higher dose of benzo rather than trying to 'potentiate' your benzo dose. Getting trashed on benzos can be messy, but it's nothing compared to the chaos what follows benzos + alcohol.
And props for actually looking for infomation/asking questions on the internet before taking the pills- too many people just think 'fuck it, ll'be sweet as' and end up in completely avoidable retarded situtations.
Also- Wellburtin (Bupropion) isn't a SSRI, it's a DNRI and is distinct from prozac, zoloft and that whole line of SSRIs. Originally released as a treatment for ADHD, it's a bit of an oddball drug that had been bounced around for quite a while. I'm very opposed to SSRIs (they have consistantly given me horrific side effects), but I found that Wellburtin did actually help with my general depression/motivation and didn't give me any of the regular I-must-kill-myself-because-I-am-a-zombie side effects other SSRIs gave me- just a bit of insomnia and indigestion. You know what's best for your mind and body, but just try to keep an open mind. Maybe it's the drug meant for you? Like it was for me ( eventhough I am off it now I did take it for around 8 months and I think it genuinely helped- if/when I go back on antidepressants I'm going )