Washer standpipe install double check

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Hello:

I am adding a washer and dryer to an apartment in a duplex. This is on the 2nd floor. The other apartment is on the 1st floor. There was a 1-1/2 galvanized vent pipe very close to the new washer location. I am moving that pipe over to my new washer standpipe stud bay.

Note: I believe that only the lower apartment kitchen sink from the 1st floor apartment is using the 1-1/2 galvanized vent that I am planning on using as a drain and vent. That sink connection point is about 10 below and 10 feet over from my new washer connection point.
Note: These is nothing upstream of my washer drain connection point. The pipe goes from my standpipe through the attic and out the roof.
Note: The washer will be a high efficiency front load washer (so less volume of water than some other types of washers).
Note: This will not be inspected and I would like to use the nearby 1-1/2 line if that won't cause problems.

Questions:

1. Is there any value in increasing the diameter of my vent pipe to 2 inch then reduce it back down to 1-1/2 in the attic before it leaves the roof?
   
   
2. I know that ideally my washer standpipe and drain is 2 inch but the nearby drain is 1-1/2. I have read many times that a drain should not be reduced. So my plan is to keep the drain at 1-1/2. Since my standpipe is only about 3 feet from the original 1-1/2” line I am tying into, even if I increased it to 2 inch, it would need to reduce back down to 1-1/2” pretty quickly. Agree to keep it at 1-1/2” or maybe do a portion at 2”?
   
   
3. If I use this vent as a drain line, will the lower kitchen sink and new washer drain ok or will there likely be problems?

Thank you!

 

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FYI: I tried to include a picture of my proposed layout but the picture does not seem to be visible . I am not sure if it is a problem at my end or the web site. I will try uploading the picture again later but I think my questions may be able to be answered even without a picture.

 

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[Jeff H Young]

1 If its a vent from the first floor regardless of size you cant connect the washer drain into the vent , 2 your washer should absolutely be a 2 inch drain . but if you are only tieing vents together the 1 1/2 vent would unlikely not be sized big enough and I wouldnt worry about it . since you are under upc code its not venting any toilets
Id explore running 2 inch for the drain concider aborting job if its too much work