Walls and Ceilings - Cracked rafters, crooked walls - remodeling

We've got a house built in 1850, and we've been remodeling a room at a time each year. Tearing out plater and lathe, furring the walls straight, running electric and plumbing (if needed), insulating and drywalling. The bedroom and bathroom went pretty well last year, and now we started the upstairs. I'm running into a few things that I'm not sure how to handle...
(there are images of the room, along with details from the questions at:
http://www.brianandheather.info/house/house.htm)

1) after tearing out the lathe and plaster, I noticed that several of the rafters are split near the walls (pics 1 and 2)...can I just toenail screws into the bottom end to keep these from splitting more (you can see some screws started in pic 2)? Can I use something to sister the rafter up (if I use 2x4s I'll have to drill each hole since the rafters split so easy. Are there any metal options for sistering rafters??
2) the walls can either be straight or plumb. I ran strings across the wall in 3 places from the large timbers in the corners, and if I fur out the rafters with 2x4s the wall will not be plumb...but it will be straight...which is better? You can see what I mean in pic 3
3) can we put R-13 right into the sloped sides of the roof (see upper left side of pic 6)? They're 5 1/2", and there are currently no soffit vents on the roof, so they're not really getting any circulation currently. Not sure if it's a good or bad idea to insulate in there?

Thanks for any tips!!!!

B

1. It appears from what I can see in your photos that your rafters are pretty much beyond repair. Probably will require you to place new rafters beside what you have. There appears to be considerable dryrot and water leakage problems.

2. if you are furring out the walls there really shouldn't be any problem getting them straight and plumb.

Do I need to sister the entire length of the rafters, or can I just sister up the 3-4' section that is cracked? Should I use 2x4s, or is there any way I can use some sort of metal strips??

Also, I suppose when I furr the walls out, I should also furr out the beams in the corners so that they're plumb, then the rest of the furring strips will be plumb as well.

Any concerns on the insulation? Do I need to put in baffles, or will it not matter since there isn't any current circulation (there is a vent at the peak of the roof, but I'm not sure if that provides circulation for the entire roof...

TIA!!!!

I would sister the whole length. I'm not aware of any way to properly repair those rafters.

Sounds like that would work for the furring out, Usually I work from the point that is furthest out and make everything plane out with that point.

I would have to say that adding baffles between the insulation and sheathing as well as adding soffit vents would probably be best. Ridge vents aren't sufficient enough for air circulation on their own. Without proper circulation you risk condensation problems.

Should we try to "jack up" or brace up the old rafters when we sister up the new pieces? Can we use 2x4s, or do we need to use 2x6s?

Thanks!!!