Yes, it will work on the 128LD. I own that tool and have used the pole chainsaw on it. Both with and without the extension shaft. Works well. Oiler works well.
Everything that I find shows this to be a solid drive tube assembly and does not allow any attachments. So I would say it will not work.
It is not recommended to add any extensions but I have added one. It does make it difficult to handle the saw due to the weight that far out on the end. It also takes some effort to raise it up and to use it to saw. You have to be somewhat careful because the extensions are just tubes that can be bent easily. If there is more than one I don't think it could be managed without damaging the extension tubes. They would probably kink under their own weight.
The TrimmerPlus 8 In Pole Saw attachment can use the Oregon Model #R34 replacement chain or look for a chain with - Pitch: 3/8 Inch low profile gauge: 0.043 In. narrow kerf, drive link count: 34 for pole saw sizes up to 42 cc
Unless the trimmer takes attachments, no.
The TrimmerPlus attachments appear to be compatible with TROY-BILT, Cub Cadet, Yard Machines, Yard-Man, Bolens, Craftsman, Husqvarna, Toro, Homelite, McCullock, and Ryobi. These brands have a mid shaft connection and clamp assembly that allows for removal and attachment of various implements. Not knowing which Echo unit you might have does not provide enough information to indicate that it might be compatible. Looking at the Echo website most of their equipment appears to have a single solid shaft, something that would not be compatible with the TrimmerPlus attachments.