If I pick one military NI in SP as a Euro superpower (which I almost never play, but still), especially with the strategy you highlight, it's aristocratic. That way you can annex an extra vassal simultaneously and slap an extra general on your armies as you conquer multiple areas at once.
As for relevance, it is important. When you're comparing the relative strength of choices, the most relevant way to determine something's strength is to look how it holds up in a competitive environment. Absent that environment, why do we care if something is particularly effective in certain contexts? You can 1-tag the world as Ottomans w/o it still for example, so what's so bad about also being able to do it, through different means that require more investment, as other nations?
Nothing, especially when in a competitive environment you don't even consider it as a realistic possibility.
EU IV is an SP game with MP functionality, not the other way around. Arguing that a feature is sound because it's somewhat balanced in MP doesn't magically make it so in SP (where it actually matters to most players--while I enjoy MP myself I don't fool myself into believing that most players play MP) -- you won't achieve a WC in MP (most of the time you'll have difficulty building a large empire as it is without significant backing from other players) so most strategies and idea groups aimed towards building large empires are automatically out as they're not viable in an MP environment.
As for being competitive, EU IV is not and will never be as it lacks the most important quality of a competitive game--predictability. If anything it's on the opposite side of the spectrum considering how much comes down to dumb luck. This makes for entertaining and often surprising games, but not competitive. Add to that that MP is plagued by houserules, turning every session into a scrubfest. Artifical rules are the signature mark of scrubs, and EU IV MP is full of them.
They do not, unless you deliberately avoid a port. You could be strong enough to ignore AE outright in every patch iteration of this game without exception. The only thing that has changed over time is how badly AE creates an artificial wall through unified truces limiting war score gains, a fake barrier I don't mind seeing gone in 1.8 because the pacing of the late game, should you care to play it out, is so much better.
Of course you can't prevent every single tag from forming or joining a coalition, but it's rarely in your interest to do so. Outside of central Asia, few tags care if you go wild feeding your horde vassal the other hordes, and those tags can't reach you if you your vassals correctly. Similarly, the HRE mainly consists of landlocked tags, allowing for feeding of vassals surrounding them, cutting off all but a few tags that you can keep for CB purposes (you don't want to end up having to no-CB DoW tags because you insulated them to much, after all). Most places in the world can be treated in a similar manner, especially now that coalitions are utterly impotent.
Feeding vassals is fine. They're your subjects and you are strengthening them. In a competitive environment, this carries risk as they can be engineered to no longer be your subject, and then you're screwed.
So you accept that vassal feeding is far too easy in the absence of competent players to exploit it, yet you believe it's working fine? Out of curiosity, how many times have you lost control of your vassals in SP due to your own incompetence, e.g., not because of some random undocumented patch change?
I agree with your assertion with allies. The turbo revoke thing is ridiculous, but the occupation transfer --> give ally crap it can't core incentive is high even without that, and it creates scenarios where you can actively screw a nation over as its ally in a big way, giving it massed overextension it can't core or imploding it through huge OE on purpose. Considering that right now you can even do this to other players (in theory anyway), it's something that should be fixed up.
But the turbo revoke isn't viable in your competitive environment--any competent players would shut down such a strategy before it could take off--so it should be acceptable then? As you said, "You can 1-tag the world as Ottomans w/o it still for example, so what's so bad about also being able to do it, through different means that require more investment, as other nations? Nothing, especially when in a competitive environment you don't even consider it as a realistic possibility." This is something many tags can do, although Austria has an easier time doing so. Of course, I'd be glad to see it go.
Being able to make allies implode is even cheesier than vassal feeding, and in MP it's even more powerful. Then again, MP is practically a different game, as it not only plays differently but comes with all sorts of weird artifical rules.