Create one closed polysurface object with several closed polysurface


I am quite new to rhino and struggling since more than a week with a rhino model. I have now all my objects (closed polysurface) which I want to use to create one closed polysurface so that I can print it… but I dont manage do deal with it.
Through this process I looked initially at non-manifold edges, naked edges, bad objects and verified that all the objects was closed polysurface.
Then, I used the trim and split to remove part of polysurface that are unwanted, I also used booleen union on several closed polysurface.
The thing is that, I dont understand why sometimes trim or split brings new unwanted edges (non manifold or naked). Another strange thing is that everything is symmetric, but sometimes things only work in one of the two sides. As a lat example, I dont achieve through split/trim operations to keep the polysurface closed, which then make the booleen union does not work. etc. Actually there is a lot of thing that to me appear a bit randomly.
I tried all things that I say on internet, but is totally stuck :(. I really hope to find some help here, sorry for this.

Thank you very very much!

model-quesiton.3dm (8.4 MB)

On the attached file, the left object is the initial one used to make it one closed polysurface, while the right one is the one after trying.

If you post it as a Rh5 file, you’ll likely get more help.

This overlaps with a question @Cam asked in a different thread and to which I responded. Closed open polysurface which openings dont have closed, planar loops of edges

@Cam Do you have experience with any other design software or similar?

Rhino works with surfaces, not volumes. Trim or Split a surface and the result will have naked edges. You can then close the surface if desired, either by creating a new surface or using all or part of the surface used to Split or Trim.

Usually the easiest way to model symmetric objects is to model half the object and then mirror it.

You are right @davidcockey, it overlapps with the second question I asked. The second question was meant to be a subquesiton of this one, in order to try to solve my problem but by asking less thing.

I will try to close this topic if its possible (I am new to this forum)

No I dont know really others design software, thank you for your help.