Milhouse:
“DaasYochid, what’s the difference between a house and any other eruv? Every eruv is by definition a reshus hayochid. If you could not make a reshus hayochid in a reshus horabim then you could not carry in a house. The answer is that min hatorah of course you can make an eruv, even a flimsy one consisting of nothing more than four strings, in a reshus horabim. But the rabbonon said if it’s a reshus horabim you need more than that. You can make an eruv, but it must be stronger than four strings.”
You are correct according to some (possibly most) Rishonim and Achronim. However, some maintain that a tzuras hapesach is not effective on a d’O’raysa level at all (such as the Bais Ephraim and the Chazon Ish).
“but lechol hade’os if you have rov mechitzos, and tzuros hapesach to fill in the gaps, and delosos across the main road, it’s OK.”
Not lechol hade’os. Some maintain (albeit we don’t pasken like them) that you would need delasos for every pirtzah greater than ten tefachim wide (Mishkenos Yaakov and a few others).