Not an expert by any means, but here's what I would probably look into (assuming these are probably used ebay phones or repair phones that you don't know the history):
Phone 1 (s=on and stuck at bootloader): Check that the PD15IMG.zip file you are using is stock unrooted 2.3.4. I'd assume the phone was stock first, then try different stock PD15IMG.zip versions (ie. 2.2) in the off chance it had been downgraded before getting stuck in the bootloader. If the bootloader is giving you any specific errors when trying to flash the PD15IMG, post them here and maybe someone can interpret them. Also, you might check if your bootloader is unlocked, because it might be an htcdev unlocked phone with s=on. In that case, someone might have tried to flash a ROM without flashing the corresponding boot.img for it.
Phone 2 (s=off no root): Can't you flash a custom recovery through fastboot, or through a PD15IMG.zip file, then flash a rooted ROM through the new recovery?
Phone: HTC Glacier/MyTouch 4g
DD ROM: ICS Stepp_aospX 1.0.0-MR2 FINAL