If there is enough room for a counterweight behind the wing then there is for a battery. The rest follows logically. Even bits of tin and control boxes weigh something. Changing the rules because something is not very heavy is not really fair. The moment of any mass moved out from the axis line is of value on this exorcise. I bet it could be done but without a car or data I cannot prove it.
You hit the nail on the head, Isetta needed a RHD version. There was no investment money so the counterbalance was created and entered history once the fourwheeler was denied trike licencing. This is the reality and has to be lived with.
Of course I would take issue with the Trienkel gearchange, which is very good if set up correctly. Indeed I would put it against that of the Isetta, which is pretty complex in its own way. A well sorted Treinkel will out perform an Isetta in most departments, including acceleration, as it has a slick gearchange? Vibration, yes, no way round that but then it does not have a huge chassis under it. I was never happy in a RHD Isetta. The best Isetta I had to drive was the Ex Phil Bowler 250 German boy.
Of course one reason the Isetta had the engine in the side was to create a short enough car to park with the door over the pavement. The early ISO made a reasonable stab at this. BMW had the choice of producing a well engineered version of the layout, or a smart in line version which could have been an advance but chose the option of modifying what was there, save cash, and sell cheap. Who is to say they were wrong. But with the need for parking lights in Germany parking end on to the pavement was not a realistic possibility. The Isetta represents a distinct trade off in design against development costs. The problem with most Microcars sadly.