So, I now just took the code from ZyinHUD (https://github.com/Zyin055/zyinhud/blob/master/src/main/java/com/zyin/zyinhud/ZyinHUDRenderer.java), and it finally works like a charm!
IBakedModel iBakedModel = mc.getRenderItem().getItemModelMesher().getItemModel(new ItemStack(item));
TextureAtlasSprite textureAtlasSprite = mc.getTextureMapBlocks().getAtlasSprite(iBakedModel.getTexture().getIconName());
mc.getTextureManager().bindTexture(TextureMap.locationBlocksTexture);
Tessellator tessellator = Tessellator.getInstance();
WorldRenderer worldrenderer = tessellator.getWorldRenderer();
worldrenderer.startDrawingQuads();
worldrenderer.addVertexWithUV((double)(x), (double)(y + height), 0.0, (double)textureAtlasSprite.getMinU(), (double)textureAtlasSprite.getMaxV());
worldrenderer.addVertexWithUV((double)(x + width), (double)(y + height), 0.0, (double)textureAtlasSprite.getMaxU(), (double)textureAtlasSprite.getMaxV());
worldrenderer.addVertexWithUV((double)(x + width), (double)(y), 0.0, (double)textureAtlasSprite.getMaxU(), (double)textureAtlasSprite.getMinV());
worldrenderer.addVertexWithUV((double)(x), (double)(y), 0.0, (double)textureAtlasSprite.getMinU(), (double)textureAtlasSprite.getMinV());
tessellator.draw();
The second problem solved itself: I stupid human did use posX instead of posY...
Yeah, my first Forge mod is finally one small step furter
Thanks for your help
momar