I understand where you guys are coming from and do appreciate the feedback. I took some pictures, I will see if I can put them together to make any sense, so that we can go from there.
From my research, I do think I understand some of the issues....
1st, I haven't really figured out the 4" cast iron vent that seems to connect to nothing. I'll crawl back up into the ceiling/attic that is next to the upstairs bath this weekend and take a look. Maybe, just maybe it is the drain for the upstairs toilet & shower. That might make more sense, I have only been in the attic space once and I didn't take long to look. I just assumed.
If not are you saying this can come out? I have access to it through a 1st floor closet and like I said I want to replace all the cast iron in the main drain line. I would love to get rid of anything that isn't necessary and it would certainly keep me from having to ensure it is properly hung prior to removing the cast iron below it.
2nd, if the upstairs bath drains into the 4" cast iron that the 2nd floor also uses, am I correct that this is incorrect because you can only have 3 fixtures on a wet vet line and they ALL must be on the SAME level? If this is correct, it explains issues I have with both the 4" cast that feeds the 1st floor tub/sink as well as the 2" line that drains the 2nd floors sinks. Which is why I am guessing it would be wrong to drain & vent the toilet into that line.
3rd, if that is the case then I must/should try to find a seperate vent for the 1st floor tub and sink. I've already decided the 1st floor toilet needs a different vent. If in fact the 4" cast line I thought/think goes only to an outside vent, does in fact vent and drain the 2nd floor fixtures, then am I good to go connecting my tub and sink to the 4" cast line I initially wanted to hook them to? Also, should I be fine taking out the AAV and just using that drain and vent. The sink would empty above the tub.
4th, My understanding is that per code, the 4" PVC line illustrated in pic #1(currently 2") that my 2nd floor sinks drain into, but wanted to convert to 4" and drain/wet vent my 1st floor toilet into, should be moved more inline with the main drain line. Currently it enters the 4" cast iron drain stack via a wye.
5th, I back to the 1st floor toilet. Would this work....I have 2 vents coming from my basement/sewage ejector pump setup. The first is the isolated vent for the pump basin. The second is the vent that is venting my laundry sink, future bar and future bath. If I were to run a vent directly off the toilet, could I tie into the vent for the basement fixtures? Would I need to increase from 2" to 3" or bigger? I realize I would have to install this connection well above the toilet to keep any backup from going into my basement fixtures.
I will try to draw a modified diagram. I know you guys think this this is a mess, it is, but I do want to do it as correctly as possible, which is why I came here.
Thanks again...