Heat welding only applies for HDPE to HDPE joints. Some "glue" method might work for the transom piece.
There is the "Stern Saver", which uses a temporary "stick on spike" to hold the piece of HDPE in place, until some thickened epoxy (probably G Flex?) which is put into 6 holes, which have ridges machined into the sides, to give the epoxy something to grip on.
There is the "Stern Mate" which also uses a thickened epoxy--don't know if there are cuts, holes or slits in the HDPE or not--but suspect there are.
I don't know if the Loctite system would work flat against the transom--the transom should be flat--the starboard should be flat--and there may be enough contact area, if you can hold the Starboard to the Transom for at least a few minutes (Although it should give almost immediate adherence, Loctite suggests clamping for up to 24 hours for full cure) However there is that warning about outdoor use, and immersion in water...
My personal experience with Starboard is that I am not convinced that a block would be sufficiently adherent. I have used a dove tail bit and cut grooves which I think would be sufficient to hold it with 5200. But in most of the cases when I put a Starboard Block on the transom, there were already screw holes, which I drilled out, undercut and filled with epoxy, then installed two screws thru the Starboard into the epoxy plugs.
Why I don't put the transducer bracket screws directly into the transom: First often you need to move the transducer to get the best image on the fish finder, second you may change transducers, third, I have seen a number of boats with water intrusion into the transom form the holes holding the transducer in place.
Example:
There are other materials, such as PVC lumber which will glue will to the gel coat and can be used instead of HDPE.
As for sealants, I feel that I am pretty much on or ahead of the curve with the proper use of sealants…It is fine if you get good prep, into a solid area of glass. But often screws loosen a bit, or they are loosened to move the transducer--and when put back--do not have a good seal--and transom rot is the result.