It can convert between different popular video formats
2. In the past I encountered a serious problem-the image of the converted video was inverted! Thus, this time I want to find a gadget to tackle this problem.
3.I want to add subtitles, in various formats, to the video I will deal with. Moreover, I want to add GIF watermark to it for both secure and artistic purposes.
PS: More features are also welcome!

I only have a partial answer. You may not get everything you want in one place (at least not for free). A good converter that handles lots of formats is Super. When I did editing (e.g. adding titles) I used Windows MovieMaker. As for the watermarks, I'm sure that you can find that doing a search.

I recommend an all-in-one video converter I often use to you.
It is called Boilsoft Video Converter, which can convert video to all popular formats.
In additon to the abovementioned subtitles, It can also add watermark to the video you want to convert.
I'm not able to answer since it has a lot of features. Here is the website:
You can have a look at it. Wish this could help u.