I have two thoughts on this:
1> Make the amount of power applied to the cart proportional to the amount of redstone power applied to the track, so if only 1 RS power was applied it would only add a small amount of speed. The downside is this would make powering small track circuits unwieldy in the absence of resistors.
2> Add the inverse of the powered track (braking track) and remove the braking effect of non-powered powered tracks. For both powered and braking tracks, with no RS power applied it would behave as a normal track. Think of roller coaster brakes...
I encountered a similar issue in my chicken vortex generator: Make a 4x4 rail square with three powered tracks on one side, drop a torch to power the track. If you place an empty cart (and push start it) it will barely make it around then hit the powered rail which will push it around again, this seems normal. Now put something (yourself, a chicken, etc...) in the cart and it begins to spin out of control, pretty quickly it will fly off the track.
I doubt it. The adult chickens resume normal behavior on quit/restart and when they finally "touch" the parent chicken. I don't know the AI system at all (MCP isn't even helpful at the moment...), but it seems like they're stuck in a state where they want to path to the parent. I would say that either the "path to parent" state needs to be aborted when the chicken grows to an adult (simple flag check) or the chickens behavior needs to be completely reset.
Also reproduced in 14w02c
Reproduced in 1.7.4, testing 14w02c now
Also happens in creative. As a control test I placed an adult chicken on the ground and fired a bunch of eggs at it, eventually it had a nice crowd of babies following it. When the babies grew up they switched to normal chicken behavior. Easiest way to reproduce is to box a chicken about 7 blocks above ground and fire a bunch of eggs at it, the chicks will swarm directly underneath.
Verified in 14w20b. RIP: Chikken Vortex Generator! Long live Chikkin Lavanator!