Hey the United standings improvement agency is still running strong. Unfortunately faction standings are not cheap since currently the only way faction standings are shared is via the epic arcs, and with 3 month cooldowns that affects the pricing. Capsuleer day reduces the price thanks to the boosters. So generally if you need lots of standings and can wait, that’d the best time to do it.
*went back through to read some of your posts. AFAIK, being -10 to state protectorate wont prevent you from joining calmil but the faction standing is what will.
There is another option to fix your standings if you dont want to utilize the USIA and that is to follow the faction standing repair plan.
You are right that when missions are shared its split 50/50 and even less the more people are in fleet. FW as long as i remembered always dropped standings of who you were fighting against as at that time Consequences of your actions had meaning.
You can also, as @Syzygium mentioned, see if anyone will share their soe or even caldari epic arcs with you (just the final missions). Storylines and others like data centers or COSMOS do not share standings. And if you can get caldari to -2 with skills, then you csn start doing l4 distribution and do your own caldari arc etc.
As far as a market character at jita, almost everyone just creates an alt for that and with skills and caldari navy standings at 8.00 your broker fee would be 1.34%