At least with Riker, we also know that it is a combination of the transport operator splitting Riker across two transport streams instead of the usual one, and a bunch of unique circumstances surrounding an ion storm. It's only been done twice, from people doing the exact same procedure in exacting circumstances.

We also know that the transporter isn't a simple clone and kill device, otherwise, their replicators would just utilise the same functionality, and we know that they lack the fine detailed resolution to recreate living matter, or computer chips with it, the result having telltale problems indicative of replication.

Scotty and Voyager would not need to rig up some hyper-complex loop procedure to keep people inside of the transporter otherwise. They could just keep the clone pattern, and put it into normal persistent storage. DS9 shows that that is possible to do that, albeit for a small handful of people per Cardassian space station. The transport accident in TMP would never need to happen, because they could just abort the transport procedure and recreate the clone from the sending transporter.

We also know that the transporter has some error correction capabilities. Scotty seemed reasonably convinced that it might have been possible to recall Lt. Franklin. Geordi disagreed, but more due to the level of pattern degradation, rather than a damaged pattern at all. Though fabricating half a person is almost definitely pushing the limits of those capabilities, it's not impossible. Those imperfections and errors are implied to be what caused Transporter Psychosis in the early days. There do also seem to be variations in the copies that come out the other end. Both parts of Kirk came out different, as did both copies of Boimler. Riker may have been the same, but we don't know enough to say for sure.

So, the matter used to reassemble is not the same matter that was disassembled.

Untrue, for the most part. We're explicitly told that the matter stream is what gets transported, with the constituent matter being converted to energy, moved across, and converted back. Barclay is held at that junction where his matter starts converting to energy, and there's a real concern that it wouldn't be possible to hold him in that state for long.

He then doubles his mass by grabbing onto another person, which oughtn't be possible if the transporter was cloning people, since the other transporter would not have received the pattern to reintegrate with. It'd just squish everything into a double-mass Barclay.