The main issue that a gerrila group is not an defensive force,
I have some Vietnam Vet friends who would rather vehemently disagree with you on that. Living history aside, though...
not everyone is in for die for the cause
A) What do you propose the purpose of Mandalorians is?
A.1) We've been spreading around the Mandalorian philosophy and doctrine and while every single writer has disagreed about
precisely what that means and is because it was developed off of one guy who explicitly was noticed for "cool factor". Among other things, we have Mandalorian Wookiees. We have enough member planets and enough Mandalorian recruiting that they could very easily be the backbone of our Starfighter Defense Corps.
A.2) Even if you set it up as a philosophy, the
Resol'nare, the baseline tenets of being a Mandalorian, from which people argue application are as follows (emphases and rearrangement mine): Armor, language,
defending one's self and one's family,
contributing to your group's welfare, rallying to the cause of the high leader (Mand'alor) when called upon, and raising one's children to follow this way as well.
Half of it is superlatively applicable to precisely this sort of scenario.
B) We haven't seen everything the success of Call to Order (
link) created.
The Mon Cala have been setting up deals with other planets in exchange for skilled engineers to head to their worlds. The Shards have spoken with more species now than ever before. And that's just in an immediate sense.
There is a quiet but palpable sense of unity forming now with your help.
Relevant results include: CNS stability dramatically increased, framework for governmental structure actively set up.
B.1) One of the fundamental elements of a government in Star Wars is "we want to continue to exist as a polity in a galaxy literally named for inter-polity violence". Setting up the CNS as a government means this will be forming.
B.2) We may be getting Mon Cal Cruisers ~15+ years early (a few turns from now once the Mon Cala have gotten results from their "Search for Engineers" diplomatic action pays off).
mass produced and acessible to guard every sistem.
C) So you're looking for something to fill a role...perfectly filled by X-Wings.
C.1) X-Wings create a very "We can't stop here. This is X-Wing Country" vibe for pirates and hostile polities.
C.2) You make an example of the first hostile polity to attempt something like this and all the rest of them think twice.
C.3) The
Malevolence and
Pride of the Core seem to be volunteering to allow us to demonstrate an example upon them.
It seems like every reason you give that it shouldn't be X-Wings creates two or more reasons it very definitely should be. Kinda like every X-Wing our enemies shoot down will bring with it two more shooting back.