You kind of get used to it frankly. Sentences still end it is just the first and last full stop will be before the sentence, i.e.:
مرحب. أنا ابراهيم وذهب إلى المسجد.
Punctuation is a bit of a complex one but as you an see here: https://area51.meta.stackexchange.com/a/14601/112766 brackets can work if they are not mixed with Latin characters.
For it to work you will need to write the entire question/answer in Arabic or English, including the numerals, i.e.: (٦٥٠)
As for the rtl option: it is a lot more complex than "just adding it". SE (Stack Exchange) was never truly designed for rtl and it seems much of their tech was implemented without thought for it and, of course, their web interface is not designed for it either.
So it s likely that it is something that will not be coming anytime soon. All rtl languages have to deal with this problem currently.