Why would Akainu have left him alive with the knowledge that he would have to kill him anyway later on? That doesn't make sense, I don't see Akainu being the aggressor until Kuzan acts first and than it would be to late already. It's not like he was prestigeless himself, he probably had a lot of supporters too, just not as many as Sakazuki with his deeds in the war.
About him going alone everywhere, that could have just be his privilege as an admiral, to conduct secret operations alone. That way he has got at least some time alone and wont be shadowed/observed by anybody else. And it's not like he has shown too much mercy towards the Mugiwaras with all the freezing to near death.
Garp and Sengoku retired so that they don't have to grapple with the whole Akainu situation. What could Akainu now do, that he hasn't in the last two years[and war], that would make them rage so much to against him? If anything, I would say they are already doing something or they won't until the final showdown of the manga. Garp should have stopped him right there[the war] if he knew, that one day even Luffy could be killed by those hands too.