The difference between 5-pin DIN and MIDI is that MIDI signal is a digital signal. It contains programmed sequences of data…. "bursts" of information. And it definitelly needs some sort of chip which understands the language and then converts those digital commands to useful action for analog domain.

DIN is analog…. and those 5 pins are used to choose instantly between the available electrical "open/closed" choices. That DIN connector is pretty much the same as those 4 jack connectors are…. but it just packs all those "lines" into one connector.

… Eclipse out => Mesa Fx Loop …..

Yep. If you want to get the preamp sound of that Mesa…

Mesa Fx Send –> Eclipse In ……. & …….. Eclipse Out –> Mesa Fx Return