Great! The only problem here is, that the aircraft I'm having problem with has
1265 polys and I really don't want to spend the rest of my life texturing it!
But as I already mentioned something is sypthomatic (keep in mind, I'm converting other's people sources, mostly designed in FSDS 2): I have no problems with the project files that had the textures already assigned and I can check and see that those were assigned as part textures. After some lighter arrangements and rescaling to meters (they were mainly designed with unit feet), I get a nicely looking static aircraft with low to modest number of polys, that compile as scenery mdl, so I can place them with bglcomp using compilation xml file.
But in case of the Saratoga fsc, there were no textures assigned, so I made my own template, started asigning textures as part textures to the project file and got a nicely looking "dressed up" aircraft. I bielieve, that if something really is wrong with my procedure, it would already show up in the design, but so far everthing looks fine. The problem is, that only after the compilation the final result in the simulation is as already described. Since already three members of this forums reported having the same problem, the explanation can't be so simple, that all three of us are doing the same mistake.
But if version 3 of FSDS works for some people and it doesn't work for some others we are doomed, because finding out the reason for this behaviour is next to impossible with millions of possible combinations of software we have on our machines. So, I'm again asking if there is someone who would be willing to try to solve my problem. I can send both the *.fsc and texture files to a person interested to help.