What I don't quite get after reading up on the mechanics is which of the factions can even potentially harm his space and make it challenging for him to hold in the long run. Mahon reduces Fed and Emp influence while boosting Alliance and Independent so both Feds bordering Mahon space have a harder time flipping systems away from Alliance to increase Mahon's fortification costs while he has an easy time converting to Alliance as both Feds do not impede that.
Our fortification triggers are not reduced by having Alliance factions in our space, but by having a majority of corporate factions in control. Our triggers are increased by having a majority of Communism, Cooperative, Feudal or Patronage factions in control. The boosting of Alliance and Independent factions over Empire and Federation factions doesn't work - it's supposed to, but extensive and careful testing in our space shows that there is no difference in their influence gains or losses.
Empire is too far away and Feds are in the way for them but even if they would ever reach up Mahon also penalizes their influence the way he does with the Feds.
Again a slight misunderstanding of how the BGS works. If you look at the "stats" page on Mahon's PowerPlay page, you'll see that currently Mahon's space is primarily run by Federation factions (730 Federation systems, vs 8 Empire systems, 406 Alliance systems and 529 Independent systems). We've been working hard to reduce the number of Federation systems in Mahon's space as part of roleplaying Alliance supporters, but doing this without thrashing our triggers can be quite tricky (and results in some frictions internally in the Alliance).
The only factions I even see eligible to flip Alliance systems are smuggling ones but nobody smuggles so the BGS impact is neglegible and one can not leverage the non-PP population into passively assisting the cause - that's before considering that piracy isn't a common "profession" either.
No. Again, moving systems away from Alliance control isn't what's important in terms of affecting our triggers - it's moving them away from corporate control. This is tricky, as corporate factions tend to be favoured in BGS (currently 34% of ALL populated systems are run by corporate factions, regardless of whether they're in PowerPlay space or not.
I get that Powerplay's immediate results are all about cooridnation and dumping effort onto PP mechanics in certain systems to prepare/expand/undermine/reinforce but the long-term impact of system allegiance changes reducing fortification costs and a lack of counter to Alliance influence gain to me makes it appear Mahon (as only Alliance faction) just has to come out on top (assuming equal inhabitance of space and equal player spread amongst PP-factions, which I know isn't the case).
To me this portion of PP seems flawed and skewed, making competition an uphill struggle due to design bias. Am I getting that part right?
PowerPlay is highly flawed, and as a result we're currently stuck in a rather passive phase of it. There's the occasional large turmoil (like Aisling just went through) which results in a few changes of the landscape, and there's long term lasting damage done by people who aren't interested in playing fair (there are ways of hurting powers that are insanely effective as they have no viable counters). There are plenty of ways that Frontier could change some of the mechanics to make PowerPlay far more volatile and dynamic, but currently it's being ignored to an enormous degree.
Now, obviously it's easy to suspect that I'm trying to dissuade you from fighting Mahon, so don't take my words as gospel, and take the time to talk with the other organized powers.