Reasons for basement plumbing setup?

[karsc01]

I'm considering finishing my basement and putting in a bathroom with a sewage pump (or grinder pump). I've been researching various aspects of it for a few months now. However my current plumbing setup seems to be a little interesting and I'm looking for comments.
I have two bathrooms in the house currently and neither is over the basement. One bathroom is over a slab and all the plumbing for it is in the slab. This seems to go right out to the septic tank. The second bathroom has plumbing for the toilet going out the wall and to the septic tank but the sink and shower seem to be routed through a 2" pipe to the basement.
So... in the basement there are pipes from the clothes washer, 1 bathroom sink, and 2 kitchen sinks only. They join into one 2" pipe that connects to the 4" cast iron outlet which goes through the basement wall. There's no house trap (that I can see), only the regular traps at the sinks. I have a few questions since this only seems to be grey water with no sewage and I may not put a bathroom in at all if I have to touch the old cast iron cleanout. It's in a difficult spot and has old oakum lead seals.
1) Is there some reason that a plumber would route the toilet and sink/shower separately? Why might the toilet in that bathroom be routed out through the side of the house (under ground of course) directly to the septic tank but the sink/shower routed the other direction to the basement and tied into the 4" outlet pipe there?
2) Could the reason be that there's 2 septic tanks? I thought there was only one but I've never had them mapped out or pumped. What would I need to worry about in this situation?
3) Would it be alright to route a sewage pump into this 2" pipe that goes into the 4" outlet? My concerns would be that currently it's greywater and no sewage and there might be some reason not to do this I'm unaware of.
4) 2" is pretty small but that's the same size as the sewage pump outlet pipe so I assume this would be acceptable. Or is that not the case because the 2" outlet for the sewage pump is forcably being pumped but once it joins the regular line it would no longer be under that kind of pressure. I'm considering spending more for a grinder pump to avoid any problems. Would this be a better choice? Could this be done or is the current pipe too small despite the sewage pump outlet size?
5) If I tie into the 2" pipe the line from the sewage pump would be the last thing to tie in before the 4" outlet. Would I want to put in a house trap before this to make sure that if any of the sink traps dry up I don't get any sewage gases backing up?
I'd greatly appreciate any advice, questions, or concerns I might be missing here. I really don't want to bring out a plumber for advice and then waste his time if it turns out it's not practical to do what I'm looking for. However if nobody here has any thoughts on it then maybe I'll just have to bring someone in.

Thanks.

[karsc01]

Well, does nobody think I have anything to be concerned about with this setup? Is the concensus that it should be a simple matter to tie a grinder pump output into the existing 2" line with a new house trap? I tried to include a picture below since my description is long and may have discouraged some people . Thanks.