•Wants a balance of the powers to be mantained. They were worried about it after WINNING against the WB pirates.
They want to mantain this balance always yet it is a fact every time 2 yonko meet the navy starts to shit bricks. See Shanks meeting WB after Marineford or when there was the fear of the Rock returing (Kaido and BM joining hands).
•Shanks has some kind of relationship with the WG and this might have had its influences. We also have Sengoku's sentence "only because it's you, Red Hair".
The translation I found says "it is fine for me red hair, I will take responsibility for this"
https://mangasee123.com/read-online/One-Piece-chapter-580.html
anyway this can be seen the other way around. Shanks is a sort of guardian who tries to matain some sort of balance as well. He didn't attack the navy despite in a state of weakness because he probably recognized that the WG has some bright side as well, for sure better than have Kaido/BM/BB profit from him and the WG fighting.
•The marines had already accomplished their objective and the one in charge (Sengoku) definitely cared about the cost of lifes.
This is true but it is true as well the marines followed Shanks threat without many questions. They still wanted to catch Luffy, who indeed demonstrated to be a pirate king seed like his brother, and destroy what was left of the WB pirates as well to show WB and Ace corpses but that price to pay was too high.
•Let's say they fight. What if in the midst of this whole chaos (Red Hair Pirates, BB pirates, Marines), they lose an admiral, one of the ultimate military weapons through which the WG enforces his control over the majority of the rest of the world? What if another (or plus) force suddenly shows up to take advantage of the situation against the government? What if both things happen? What if the WG suddenly has to fight three forces against them? Does the WG wants a scenario like that?
That is why they obliged to Shanks threat. They were already damaged when suddenly a fresh yonko and crew appeared before them. If instead of Shanks it was Kaido with the beast pirates or BM with her sons then I don't know how it would have ended but the navy would have been destroyed or at least on the brink of it (even because they had prepared for WB a plot to weaken him through Sguardo and the best battlefield/fortress they could use while the new yonko was a totally unexpected guest and even the battlefield was a mess by that point). Anyway Shanks too didn't want to fight cause it would have been a bloodbath either way.
The indication I have always reached from the manga is that Marines HQ + Warlords = 4 yonko crews (the balance of powers) and that the MF forces are WAY above a single Yonko crew. I think that @Van made a very good thread about this as well.
That is impossible. If the navy is as strong than 4 yonkos combined then they could easy eat them one by one since they are enemies amongst themsleves as well. The truth is that this balance can work because the yonkos hate each other and indeed like I said above each time 2 yonkos meet the navy shits bricks cause they know that may spell doom for them.
Let's make a scenario. The 4 yonkos of 2 years ago (so WB, Shanks, BM and Kaido) decide to erase the WG from the world so they attack the navy toegether with WB. Is there anything the navy can do? Even if we suppose all the warlords are ready to die for the navy (and they will laugh at this idea) there will be nothing left of Marineford after a swift storm. They afterall had to gather their best, prepare the battlefield and plot behind WB in order to accomplish a difficult victory.
But well, to each their own.
And we can agree to disagree.