I do not think anything you described requires a formal Alliance with its associated investment (an easy billion isk). Many of the points you provided can be done without the need for a formal Alliance (and may provide corporations with additional flexibility), by establishing shared channels (replicating Alliance chat) and mailing list (replicating Alliance mail).
The mechanical benefits (from my understanding) of a non-sov alliance are largely shared standings, wardec consolidation (likely w/o a cost savings)*, and no wardec limit.
Another advantage is that there's no wait to war-dec someone. My understanding is that you need to have a corp vote for a corp to war-dec someone, but an alliance can do it without the vote. This is enough of an annoyance (and extra 24 hour delay) that some singleton corps create alliances just to avoid this.
*Alliance, say CVA, decides to wardec all EM - total cost 50m isk. Say instead EM was a loose association of corporations and CVA wardeced all those corporations - total cost 350m (7 x 50m/corp).
Actually, it's worse, since I think war-dec fees stack, so 7 wars would be 50+100+150+200+250+300+350 = 1.4B (I think - not in game to check, and have never personally had to calculate it).
However, I'd expect that what a hypothetical enemy would do is they'd war-dec one corp of the loose association, and not the rest. Stomp the singleton, and then war-dec the next. The only way for the other corps in this "federation of corps" to get involved is to counter-war-dec, and wear the increased costs. For Corps who end up in high-sec wars, Alliances really do help defend against aggressors.
And for all that, EM war-dec's pirates about as often as people war-dec us.
Late last year, several EM corps did leave the alliance and join the Matari Militia. We tried the "shared channel" and "shared mailing list". The lack of shared standings was actually a significant pain, since EM has an NRDS engagement policy, and frequent standings changes, which is a job that is amenable to economies of scale (There's no more work to set standings for an alliance than a corporation, yet all the corps in an alliance can share the standings).
After a month in the militia, most of the members were very happy to get back into the alliance, and reluctant to leave again without good reason.
I think that you underestimate the value of the psychological effects of being part of an alliance. I think that EM finds that psychological effect of great value, and once the alliance is formed, well worth the monthly price.
If a group of Federation-aligned corps want to pursue an alliance, it is likely better that they first attempt a pseudo-alliance with a set of defined goals either in regards to FW or some other shared goal.
I think this is a reasonable piece of advice, but I wouldn't do it for too long.
If you're looking for something similar to EM, I probably would try to tie your reason for existance to something other than FW. It's a bit broken to RP around.