Replacing wet return on 1-pipe steam system

I’m doing some demolition in the basement and with the walls open I’m considering replacing the 100+ year old wet return pipe.

IT runs about 60’  from the front of the building to the back and then over 15’ and then under the slab for 8’ to the boiler.

The pipe looks pretty rough.  The pipe is sitting on the slab and someone framed double stack 2x3 right on top of the pipe.  All that framing is coming out.  I’m going to replace it with metal studs.  

1. If replacing, should I replace with copper type L or use black pipe.  Copper is going to be MUCH more expensive.  Black pipe will corrode faster.  But who knows how long/often the steam system will be used.  I’m going to use it as a backup system for heat, when it’s too cold for mini splits to function.  (It’s a well insulated building)  When the pipe goes under the slab I plan to use copper.
2. Does it have to pitch 1/8” foot toward the back of the building?  There’s no way my slab is 60/8” lower in the back.
3. I’m going to install a ball valve and hose connection in the front of the building so I can flush the system.  There is a similar setup at the boiler.  
4. Return pipe sizing - I think the return pipe is 1 1/4” (will measure with calipers).  Is that size adequate.

Boiler Burnham IN6.  15 radiators.  3 family building. 

Any advice or tips greatly appreciated.