Hopefully I can make sense here and not duplicate what Sam and Jerry have said. Forgive me if I get long-winded, as I have thought a lot about this kind of configuration and have way too much to say.
1)Flat-felled is what I like, and I try to orient the fold of the seam so that it sheds water rather than collecting it. Sew a sample seam in some scraps and ply with it and you will see what I mean. And this leads to :
2) If you can arrange the seam layout so that your mesh is sewed into the seam that you already need due to fabric width, that is ideal so that you have one less seam to seal. The question is, will that seam be far enough down the slope to suit the bathtub size? Maybe and maybe not, depending on your precise configuration. You give a dimension of 100” for length, but is that at the bottom of the fly? I would guess it is, since that’s longer than I would think you’d want for the floor, but I could be wrong. Your drawing shows the mesh running horizontally out to the edge of the fly from the top of the bathtub floor. Doing that may mean that the tub is suspended only on the long sides (door side and back) and the short sides will tend to flop down, unless you can set up some tension along that short side at the top of the tub (your setup can have tension at eh edge of the mesh where it attaches to the fly, but cannot at the top edge of the tub unless you add a connection from top corner of tub to either bottom corner of fly or some point along the main roof/vestibule ridgeline – but that may not be possible if you do as suggested by Sam and relocate the mesh top of wall attachment to that seam. The other approach is to have all edges of the tub supported by having your end mesh come down vertically from where it attaches to the roof – that allows it to support the edge of the floor tub, and may be helpful in terms of where your roof seam is located due to fabric width. I realize that ins some situations condensation running down the roof can meet the vertical mesh and run down it so that it is now inside the tub, rather than running toe horizontal mesh where it should drip through. So that is a point worth considering, and I have thought that some sort of tensioning setup – say a couple vertical shock cord connections – that hold up the edge of the tub, while mesh runs out horizontally or even downward to its attachment with the fly, might be the way to go. But more complex of course.
As to the actual stitching of the mesh – it can be a bear because it is stretchy, especially on the bias, so holding it in position is the hard part. The mesh does not tend to unravel in my experience, so seam detail are not very important – even just a single line of stitching often exceeds the strength of the mesh, so fancy stuff is not necessary. Taping the mesh into position may be useful, if masking tape will stick to your fabric – finding tape that sticks to silpoly well enough is not easy. Pins I like to avoid with waterproof fabric, since they obviously leave holes that you then need to seal; but since you are probably sealing the seam anyway that may not be such a big deal.
3) I think Jerry’s peak reinforcement plan is spot on, with the caveat that if you have mesh wall coming in at the seam that leads to the peak it gets more complex. If that is the configuration you end up with I would consider not sewing the mesh for the last 6inches or so up to the peak in order to leave room to do the peak reinforcement. Than you could just glue that last bit of mesh with your seam sealer, or throw in a few stitches thru the reinforcing cone that will also hold the mesh away from the peak to keep it out of conflict with the pole.
4) What you’ve heard for the curved zipper on the mesh sounds like the way to do it. Trying to get alignment any other way is sure to fail. Of course you can go Sam’s way and avoid the curve, but it’s still tricky to get alignment with a zipper in mesh, and if you like the arched door, as I do, go for it. When you cut the mesh, try to cut right along the center of the zip, and then you can fold back the mesh at your original line of stitching and topstitch it out of the way so it won’t snag.
5) Sounds like what you mean is not so much reinforcement as effectively small guylines for the bathtub. This is feasible but tricky to get right – all the angles of the lines have to be right so that all can be in tension, and all the lines should be shockcord in order to have some flexibility to accommodate variations on pitching. I think I would avoid all of this and just let the floor hang loose from its top edge, but I would make efforts to achieve some tension at the top edge (see #1 above) so that the top of the tub stays well off the ground. Yes it looks a bit sloppy to have the floor hang loose, but it is functional.
If I was going to try to get the tension ideas to work on the bathtub corners as you suggest, that photo is not much use, because as Sam has pointed out, that’s a rather different configuration that what you envision. And I would recommend that you add some stiffeners to the corners, vertical rods of something very light and skinny – carbon maybe? – right in the tub corners from bottom of tub to top – since that would help make it work by having a compression element . But again, I do not recommend that approach in the first place – just saying if you do try it, that might help.
On a more general note, I will have to respectfully disagree with Jerry regarding catenary cut – I think you want it on all edges of all panels of the tent for the best, tightest pitch, regardless of whether the seams are with the grain or on the bias. But I heartily agree that where you can you should join grain to grain or bias to bias for best results. I know that’s not always possible but it’s worth aiming for in general.
If you can get some cheap fabric and make a half size model of this thing you will learn a lot. Every tent I have sewn would have been much better on the second try, as I always find things that don’t work quite as I expected them to. Drawings are great but they do not show where the tension is and where it needs to be to make the tent work.
Another thing you can do is the mock up the entire configuration with string, the string being only the ridgelines of course. This could be very helpful with sorting out your floor to roof connections.
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