Advice on running waste pipe

Hi there

I am planning for running a new waste line from the second story of my 1898 house to the crawlspace about 12 feet below. I will be running this through a stub wall which will give me a maximum diameter of about 3.5 inches. I have "as-built" architecture plans drawn up and will be applying for the requisite plumbing and building permits (I'm putting in a new bathroom).

So a few questions, the pipe will have to do something close to a 90 degree turn at the bottom of the 12 foot drop due to a major structual beam below the stub wall, and then extend about 20 feet to the soil stack. With a 1/4 over 4 inches drop, this works out to about 5 inches drop over the 20 feet span.

Q1. Is this angle too sharp? Would a long 90 degree span be sufficient?
Q2. Is the "new" cast iron pipe of an external diameter that can fit in this stubwall? Can I (should I) mix cast-iron for the drop pipe and then use ABS for the span (due to weight?)
Q3. Attached is a shot of my soil stack. Plan A is to cut the stack in the top span above the central Y, and put a new Y in in this place. Does this seem reasonable? Or due to the shortness of available pipe, should I chop the pipe lower and replace the whole shebang?

Q4. At the top of the pictured soil stack on the top floor is the Y tha went to the former toilet. I have since removed this but the Y and toilet mounting is still in place. How should I cap/dispose of this?

Thanks very much in advance
-Nick

since im up ill give you my opinion. others will be able to give facts.
1/4 inch per foot is sufficient
to turn horizontal you need a long sweep y with a cleanout
should run pvc or abs the whole way from new bath to cast stack
if my house i would replace the "shebang" if you have a decent spot to snap above the floor. either way you have to support the cast above the cut. i could be wrong but I think im close.

I'd run the cast between the floors, it's quiet, I replaced with pvc and its very noisy, even with insulation and foam...

cast iron

the cast is quieter between the floors..

the 90 elbow will work just fine either on the
top or on the bottom

as far as fall 1/4 inch upstairs will work great

in the basement it wont really matter too much at all
because its going to be like niagra falls dropping
down from the second flooor..