The answer to all your questions can be positive-ish.
You can use two 100 steels- or one. It is not as comfy as two Al cylinders, but doable. Actually, one steel cylinder may be sufficient for open water rec dive.
Formal training: it is not that you need to learn all the basics of scuba. You may find that the adaptation to SM is quite easy and smooth, and you can learn about gas management, sharing etc. by yourself. However, setting the gear correctly, having someone look at you and help you trim things etc. will make it easier and with more confidence.
Using back plate is the trickiest one. It is possible to use either steel, Al or plastic. Especially if you are open water diver and not going to get wedged in tight cave passages or wreck. The wing as is will get folded like Mexican tacos without the cylinder in the back- they are designed to hug the cylinder and without it they completely fold. It is not only ugly but may pose problems including difficulty to empty air. Some decades ago there were not many dedicated rigs available and divers had to diy a solution. Some ideas you can find in this very forum, many in muck diver's forums as well. My solution at the time was to use a "No STA"- no single tank adpater. Instead of STA I used a broader plate and the wing was sandwiched beyween this and the BP, could not fold. The bungee for cylinder valves is a no brainer, and you need some form of buttplate or solution to hold the cylinder.
So, the above has an advantage only if you like to diy some stuff and want to experiment or experience without committing to new bcd, etc. Is it optimal as a dedicated bcd, or
good as a permanent, long-term solution?? Nope, it isn't.
