Tying new half bath to existing drain

I'm in the planning stages of a half bath upstairs and need advice on how best to connect the new drain to the existing drain.

The existing drain is a 3" ABS line that connects to the closet flange of the downstairs toilet, drops vertically a foot to an elbow, runs horizontally 2 feet to another elbow (which has a 2" heel inlet connected to a vented horizontal branch), and from there, horizontally another 3 feet to a vented stack.

Can I remove the elbow under the toilet, replace it with a combo, and connect the half bath there?

New bath fixtures will have new vents and drain would be basically an 8' vertical run from upstairs to a combo with cleanout, then a 12' horizontal run to said combo under the downstairs toilet.

Code here is International Residential Code.

I apologize for the lousy drawing I'm attaching but I hope it helps illustrate the above somewhat.

drain

The IPC is very lenient, but I doubt that even it would allow that connection because it destroys the venting for the downstairs toilet and could cause its own set of problems.

So is there a way to make a connection there and preserve the toilet venting? Would it work if I added a vent at the toilet? Or should I run the new drain all the way over to the stack coming up from the building drain?

drain

Either one, but the toilet vent would have to be done properly.

thanks

Ok. I'll leave that toilet alone then and go over to the section coming up from the floor. That'll give me the opportunity to add a cleanout there anyway.

Thanks for your help!