Additional info: gta.loadTXD seems to work only on generic.txd, player skins (but you neeed to load default ped dff to make the skin replace smh) and cars. But not from textures from gta3.img (trees.txd, trees2.txd for example.)

Sorry for double posting but heres images for further analysis and better understanding of the problem.

You need to apply the TXD before the DFF, or the updated texture won't show on it. In the snippet from your screenshot, you're loading the DFF first.
This is also why you're having to re-apply the DFF of the player skin ... it needs a "refresh"

Same thing even after applying the txd before dff. And sometimes the object turns into white boxes. And yes, im using the correct dff and txd. In singleplayer it works fine.