Not disagreeing with the substance of what you said, 43, but I do think it’s worthwhile to make the overarching point that this is another circumstance demonstrating why knowledge of the specific systems fitted to the specific aircraft you’re flying is essential. You touch on the reason in what you say about the destination of the return fuel line. Further, this statement from Fred is a bit misleading: “Beechcraft have a fuel return line so with the mixture in idle cut off, running the electric fuel pump for 60 seconds, fuel runs through mechanical fuel pump to control unit and back to tank cooling them off.”
The FCUs on injected TCM engines have a return fuel line. That’s because the mechanical fuel pump is designed to always provide more output than is required for the power setting chosen by the pilot. That applies to FCUs on injected TCM engines whether they are fitted to Beechcraft or any other aircraft brand or model.
Then we have different systems on different serial numbers of the same aircraft type. For example, the return line from the TCM FCUs on some of the early serial number Bonanzas went to only one tank, irrespective of which tank was selected on the fuel selector. That gave rise to the risk you identified. The return line on later serials goes to whichever tank is selected on the fuel selector. That risk is removed on those serial numbers.
Some Bonanzas have a two speed electric aux fuel pump. Some have only one speed. The pressure out of the single speed pump or the two speed pump on ‘HI’ can be sufficient to force fuel through the FCU even with the mixture set to ICO. That results in dangerous puddles of AVGAS when ‘the electric fuel pump’ is run for 60 seconds. I have seen it first hand.
(Research into the specifications for and maintenance instructions for the ‘standard’ FCU on an injected TCM engine showed that the only ICO requirement was that when the FCU is set to ICO on the test bench it lets through less than ’20 drops per minute’ with an input fuel pressure of 10 PSI. The pressure out of the single speed pump or the two speed pump set to ‘HI’ is waay higher than that. Another discovery – which should have been durr obvious but I’m a bit slow – is that the electric aux fuel pump is an airframe component and not an engine component. Why’s that important? Because the engine manufacturers don’t do anything different to e.g. an IO520BA fitted to aircraft X versus aircraft Y, but those aircraft can have different fuel plumbing and electric aux fuel pumps.)
And then there are different kinds of mechanical fuel pumps fitted to the same engine. Some IO520s have an EDP that is connected to the mixture control along with the FCU. Some IO520s have an EDP that has no connection to the mixture control.
In relation to residual fuel after shutting down using the ignition switch, my experience and observation is that on a hot engine any residual fuel will quickly boil off or otherwise evaporate. On an IO520, for example, the valve in the manifold (‘spider’) shuts off fuel to the injector lines when the input from the FCU reduces in pressure due to ICO at idle. But then you can physically hear the remaining fuel in the injector lines boiling and hissing in the lines and venting through the vents on the fuel injectors. The carby mounted in the updraft configuration (e.g. on my 0360) quickly ‘heat soaks’ after shut down. (It's the heat soaking of the EDP on the injected TCM engines that is the primary cause of their hot starting problems. The fuel evaporates in them.)
Cold/er engine? I’d be very wary. But we should always treat props as live and deadly, anyway.
(Don’t get me started on ignition system knowledge…)