I think much of your frustration will be alleviated by simply practicing together...alot!
There won't be a lot of "jumping in" at the beginning, if you all practice on a specific introduction. In most cases, the bass player plays for all types of song, just as you do. But maybe he needs work on taking it down a notch or two in slow songs. Again, something that can be discussed in band rehearsals and practice.
In your rehearsal times, maybe its best to just try to imitate note-for-note, whats going on in the original/recorded versions of the songs you are working on. Try to adjust your dymanics, entrances, exits, etc. to match the recording. That will give you some place to start from.
There are times when just a keyboard is needed, say when you are simply improvising, or playing talk music, etc. and the pattern you are playing isn't intuitive...then yes, to force himself into the situation would msot likely clash with you and not sound appealing.
So where there are instances where it should be just keys, in most instances, you both have got to learn to co-exist with each other...again, more practice will help. As a keyboard player, are you banging out bass notes with your left hand, and it appear to be clashing with his notes? or do you completely leave the bass line for the bass player, and you are doing alternate things with your left hand? In that instance, it may sound like he's messing things up, but in actuality you not "relieving" your left hand of running the bass is what is causing the clashing...and trust me, he is getting just as frustrated at you for running that left hand, as you are of him playing...LOL.
Do you need a music leader? I always thinks it helps to have one person to give direction, that way everyone is on the same page. But that comes with responsibilty...you've got to be familiar with everyone else's part so that you are qualified to make suggestions to them. Its more than just telling someone to play softer, or telling the drummer to slow down. Its about being familiar with the song, and with your band, knowing what the song expects, knowing the capabilities and limitations of your band, and making it all fit together to make one orchestrated sound.
