0a) I don't think there'd be much difference between plastic line and silicone hose. The nice thing about fresh hard line is you can see it get dirty to do something about it.

0b) The EDS module is in the cubby behind the battery. There's a MAP sensor somewhere in there at the end of the line that tees into the ALDA signal line.

1) 90-91 2.5s have an overboost switch that triggers at 1.1 bar or 16 psi. 15 psi should be fine in bursts but don't hold the pedal to the floor up Pike's Peak without a pyrometer. The overboost switch attaches to the manifold within an inch or so of where the ALDA gets its signal so that's where MB felt it relevant to measure pressure. That's not to say that's an appropriate place for the wastegate actuator to source its signal.

2) That's more the intention than an observation. Back in the day, turbo wastegates were fully mechanical in that compressor outlet pressure pushed the wastegate flap open to reduce manifold pressure. These days trees are things we hug rather than live in. For more precise wastegate control, the pressure actuator gave way to a vacuum actuator. The vacuum actuator works with a wastegate lever that's twice as long as in a pressure system and operates on modulated vacuum signals rather than a simpler on-off signal. The added parts and pre-standard OBD controls means it's a PITA to figure out why EDS limits boost. Rather than spend time and money figuring it out, many convert to simpler mechanical boost control.

3) Yes - a pressure actuator pushes, a vacuum actuator pulls. But think in terms of opening and closing the wastegate. I've never worked with a MB T25 so I can't comment on geometry. Whether you adjust rod length or reposition the actuator seems more a matter of convenience. When I converted the T3 in a '93 SD, I swapped over the actuator *and* lever from an '87 SDL because I wasn't sure the actuator would work properly with the longer lever from the vacuum setup. It would seem from the number of successful T25 conversions that lever length isn't an issue.

Sixto
MB-less