I haven't experienced any of the memory issues you describe. I have heard complaints about memory utilisation in previous versions of Miui, but I haven't seen evidence of major leaks.
You don't have to register a Miui account to turn on developer options - you just tap "Miui version" in the "about phone" dialog seven times. It's a slightly eccentric UI decision, but I understand that it's intended to protect naive users against shooting themselves in the foot. You do need to register a Miui account to unlock the bootloader and install a custom ROM, which isn't an entirely unreasonable security measure.
You don't have to register a Miui account to turn on developer options - you just tap "Miui version" in the "about phone" dialog seven times. It's a slightly eccentric UI decision, but I understand that it's intended to protect naive users against shooting themselves in the foot. You do need to register a Miui account to unlock the bootloader and install a custom ROM, which isn't an entirely unreasonable security measure.