No reason legionella should be a problem, as that is bred in the storage tank and stagnant water.
No; it multiplies in any water that provides a suitable environment, mainly between 20 and 50 degC, not just storage cylinders and not just stagnant. It inhabits the biofilm and the limescale deposits on the inside surface of pipes.
As an example, a DHW system was modified and a branch cut off and capped. The engineer had the capped tee cut out and straight pipe inserted. I thought that was OTT; he produced documents relating to an installation that had persistent reinfection by legionella until a few such capped tees were removed. The 50mm or so of dead-leg provided a safe environment for legionella where it survived numerous attempts to pasteurize the system.
Installing a towel rail, a heat emitter, on a DHW system is just daft. It was done in the old days, but they didn't know about legionella; they thought asbestos was great stuff too.