Well, it got hard again. I actually needed an extra main signal inlet, so, when I added it, I managed to make it work and build and stuff... but now, if I  change the number of channels (that is, if I recreate the object with a different argument), Pd can freeze or blow up... so I screwed it somehow. Here's the code: https://github.com/porres/pd-else/blob/master/classes/mtx~.c - I don't know if it's something to do with the "free" method, or something inside the "dsp" method, that's all I can think but I've also already done all I could take a wild guess on.

On the other hand, I made progress with my previous way of trying to do this. Instead of a split and mirrored output, say 1 2 3 | 3 2 1, I'm now able to get an exact copy of the input, but reversed. Say the input is "1 2 3 4 5 6", I get "6 5 4 3 2 1". 

Ok, so I see a way around would be to have a temporary array and rearrange things over. Well, I tried all possible combinations to not have that inverted result, nothing worked... so either I missed something or it's just that I really need a temporary array to rearrange things. Now, I also tried that in every way I could take a guess, but failed miserably, here's the blunt truth, I can't really code, so I need a pretty straightforward example for me to work on and develop on it, like: https://github.com/pierreguillot/pd.dummies/blob/master/src/leeloo_tilde.c - or also some fix to this code on my repository: here's the issue and link to the code:  https://github.com/porres/pd-else/issues/161

Yeah, I know, if only one could give me a hint and I'd fix it myself, but sorry, I'm not there yet. So I hope someone can give me a fix in either of the two attempts.

By the way, the end goal is not to have this silly multi channel gate object, but more of an automatic fade in/out for multi-channel files. I got all the rest worked out, but I need to get over this obstacle... yeah, I would know how to do this as a patch, but, well, I'm trying to learn how to make externals ;) 

thanks