Long Teng now blamed the excessive paint on the edge of the cover and downtube that prevent the cover to be locked securely, want me to use a rasp to grind off the excessive paint on the edge along the cover and the downtube opening
It's an OEM frame that does zero marketing, you're not dealing with Trek. I really doubt they developed a frame with a closing mechanism that doesn't work, starting with the fact that argon 18 has had these things on their frames for years (I have a friend with a TT bike from them and it's got such box too).
So, I don't think it's unreasonable to use a jewellery file or sand paper to grind some paint. I bought a whole set of jewellery files because frames often need some work to facilitate routing, bars especially, it's standard practice really.
If you use sand paper or a soft sanding block, you can easily grind off paint following contours. Any DIY shop will have these and they should cost 3 bucks. If you don't want to use sandpaper, get a Trek