A., it sounds like you are having a frustrating time :)
He continues the behavior pattern because it works for him, even negative attention is attention. I think the issue is that he is used to having you constantly, and in the afternoons, you are suddenly not there and he's thinking "Mom, what gives, you are supposed to entertain me!"
I have two suggestions. First, start making time in the morning when it is just the two of you for him to have alone play. It may only be for 5 or 10 minutes to begin with, but it is a start. You can be in the room, but do not actively participate. As he gets used to it, expand the time and distance yourself so he can learn to play by him self for a period of time (20 or 30 minutes).
Once he realizes that he can have fun without you directly entertaining him, you can put him in the same situation when it is time for the older kids to work on homework and he won't be depending on you for his sole entertainment.
Now, to the screeching, you can try to ignore it for a time...it will be hard, but if he realizes that he doesn't get what he wants by screeching, he will eventually stop. Reward him with praise, hugs and such when he finds positive ways to communitcate. Show him how you want him to communicate...but remember he is still a babe :)
Is it possible to have the older children rotate in their homework time? While those with shorter assignments are working, let the others play with the baby to distract him' then switch it around?
Good Luck.