Plumbing Questions - Convert Upstairs Laundry Room to Bathroom
I'm hoping to get some advice from all of you relating to a remodel I'm looking at doing. I currently have a second laundry room on our second floor, which I would like to convert to a bathroom (we don't need two laundry rooms, but we need another bathroom). The existing laundry room has a sink/vanity and a washer/dryer. The sink and washer each have their own separate drains. I would like to replace the washer with a new sink/vanity, replace the existing sink/vanity with a corner shower, and add a toilet. The main problem is that the two separate existing drain lines (from the existing sink and washer) are 2". That will be fine for the shower and sink, but I believe the toilet should at least have a 3" or 4" drain. But there isn't a 4" drain anywhere near this room that I can tap into.
My thought to solve this is to use a macerator (grinder/pump) toilet - behind the wall from the toilet - whereby I can grind up the toilet waste and then discharge it into a 2" drain line. I'm looking at a Zoeller QuickJon System, which will allow me to place the grinder/pump box behind the wall where the toilet is. From the grinder/pump box, I would pump up the wall, into the 3rd floor attic, across the top of the room, and then gravity feed down the opposite wall and tap into the existing washer drain. The Zoeller QuickJon System has its own separate 2" vent that I will run into the 3rd floor attic and tap into the existing main vent line (which goes out the roof in a 4" pipe). The shower and sink would both drain together into a separate drain line, which would also have its own vent that taps into the existing main vent line in the 3rd floor attic.
I'm fairly experienced in doing a lot of my own DIY projects, but I know enough to know what I don't know with this proposed project. I've attached a JPEG diagram of my proposed plumbing system, and would like to get some input on whether this system would work, and if not, what tweaks I should make. Additionally, I have a few specific questions:
1. Will the combined vent for the shower and sink provide adequate venting? Will this technically be a "wet vent"?
2. Is there any problem with pumping from the grinder/pump box up the wall, into the 3rd floor attic, and then have it gravity feed down into the existing washer drain line on the other side of the room?
3. The grinder/pump box will be in a second floor attic room (not heated) - any concerns with freezing? I live in NW Arkansas, where winter overnight lows can get into the teens. Could I build an insulated box around the grinder/pump box to remedy that if it's a concern?
4. Do I need any check valves anywhere in the system?
5. Any other problems or concerns I should be aware of?
Thanks in advance for your help/direction!