You are mostly correct. At least one of the biggest problems with pipes was that the transfer of heat between pipes and its contents was based on the surrounding material. This can been seen here with obsidian pipes not changing temperature at all in a vacuum. Since gasses (even hydrogen) have a much lower thermal conductivity compared to solids and liquids, heat transfer was slower than it should have been. I'm not sure when, but this was changed in the last few days on the live patch. If I run that save now (on live, not beta), the pipes cool down in seconds.
Also, pipes do not transfer heat between themselves, like power cables but unlike tiles. This means an empty pipe is useless at radiating heat and never changes temperature in a vacuum even when connected to extreme heat or cold. This is probably due to performance optimization as heat conduction is one of the few things that has to be recalculated every step for every tile on the map.
I agree that the new formula should be c(average) = 2*c2*c1/(c1+c2).