1) You're confusing the terms: payload, usb, etc...
2) To have AVCHD HDD playback you need a jailbreak device with syscall8 support (i.e. hermes 3/4/4b). Older payloads with syscall36 support only or PL3 with syscall35 only will not work.
3) To initiate AVCHD playback you need ANY FAT32 formatted USB stick/USB HDD connected to your PS3 - it is needed, because you need to initiate plaback from XMB VIDEO tab and there you MUST have USB/SDHC/MemoryStick connected to have the option.
The USB you connect will be used as a mount point for AVCHD HDD structure. multiMAN will create AVCHD/BDMV with required files if not present.
4) No it doesn't defeat the purpose of all that, because you can have a 512MB USB stick connected and play 20 different AVCHD HDD folders, 15GB each.
5) Using internal HDD for playback may dramatically improve playback performance for movies with high bitrate and HD audio, since external USB sticks have low transfer rates, compared to internal HDD.
In the next release I'll add SDHC and MemoryStick as AVCHD HDD mounting points (that is for 1st gen consoles like mine with 4USB+SDHC+MS+CF slots).