That accident happened just a couple of years after Strickland arrived in Melbourne, no doubt still heady with the success of EMU operation in Sydney. MU could have been an answer to that risk but the decision was obviously made for single cars due to less traffic to handle.
I think that's also the prosaic reason that the R/R1s were not designed for MU. At that stage they were not envisaged as front-line cars for the busiest services, that role still being the domain of the crossbenches. Ironically, however, the R/R1s exclusively operated the busiest tram line in Australia, the Watsons Bay, and accomplished it by the tried and true (George St) "moving platform" method, the continuous conga line of trams!
I reckon with that service, the reason for using the comfortable corridor cars was the more exclusive clientele along the line. Class distinction or an extra special effort to keep them from driving their cars, who knows?