What was done nearly 100-years ago was built to much older codes, if codes even existed in your area or were enforced. FWIW, any run needs a minimum slope, or it will have problems. All fixtures need to be vented (with a few special exceptions) individually. Newer fixtures generally use less water and move things faster to compensate for the long, slow drain of things (especially toilets an old one might have drained 10-gallons with each flush, a new one can be as little as 0.8g). So, to keep things working with modern fixtures can mean you need to bite the bullet, and do it right based on what has been learned in those last 100-years.
FWIW, any copper you cut out, take to your local recycler - it's worth some major money, don't just trash it. CI is still the premium drain material (galvanized steel should never have been introduced!). Changing CI to something like PVC or ABS (depending on what's available in your area) will be faster and less expensive, but drain sounds you never heard are likely to become a common occurrence, and maybe an annoyance. New CI is usually done with hubless fittings, so no lead, no oakum (but it's still needed with old stuff without more drastic operations) and is fast and reliable. Cutting old CI can be problematic sometimes - it can shatter, especially if the snap cutter is dull. Cutting it with a saw can be done, but you may need a few saw blades.
Old Story short..doing some updates..The tub/shower to the left of T wash basin to the right.. Wrong T right ? The direction (2nd pic from the top is better) is messed up..
