The rules for pushers and drivers are clear - there is no way their roles could be filled by alumni without it being obvious and obviously illegal, which leaves us with the mechanics.
To me, the tasks of the mechanics are the science, to figure out how to go fast, the engineering, to design something fast, the manufacturing, to build what was designed, and the operation, to get what was built to the start line on raceday.
Having alumni involved in just the science is ok, because anything they know or learn has to be passed on to the students in order to get used in the race.
The usual way to learn about buggy engineering is to look at your older buggies. If your alumni have learned something since their student days, I don't see why they shouldn't tell you by dropping a set of plans in your lap. It's your choice whether you just build it (or clone a previous buggy) or whether you learn from it and incorporate the ideas in your own design.
If you outsource the manufacturing, students either have to do the engineering to fit around COTS parts, or to come up with the plans for the machine shop or welder. The cost means that (as far as I know) no team can outsource more a small fraction of the work, and as long as it is for a specialty that no student could reasonably perform (welding, high-precision machining) I don't have a problem with it.
Since we expect pushers & drivers to be students, I expect student mechanics to handle getting their buggy on the course for freerolls and raceday. Alumni can provide advice, but shouldn't be the brains of the operation or an extra pair of hands. Obviously, a restarting team needs someone to show them what they need to do, but the team shouldn't need help the next year. I'm willing to overlook the unfair advantage because a restarting team is unlikely to be competitive, and so I'd rather they just be safer while not changing the outcome of the race. If the team is competitive, then the advantage is affecting the race, and over the line.