Ordinals are used to describe something's order in a sequence. These are the "first, second, third" words you have mentioned. However, numbers by themselves can be used to denote points on a list.
Therefore, we should use ordinals when describing things to do in sequence:
First, we need to till the soil. Second, we'll plant the flower seeds. After that, it's just a matter of waiting.
On the other hand, if someone has multiple, separate points to address, they might verbally number them for emphasis:
Speaker A: What's wrong with you?
Speaker B: Well, for one, my feet are killing me after standing around all day in these heels. Two, I'm starting to get a migrane. Three, I just found out the train is out of service today so I'll have to pay for a ride home.
Ordinals can be used in the second example, too, but it implies that the points are listed in order of most important to least important. Adding -ly to the end just turns the ordinal into its adverb form.
For me the text "in the last two weeks" means not more than two weeks. I might write "in just over two weeks" to indicate a small number of days more than two weeks but less than three weeks. For the purpose in the question I might write "I have uploaded 5 videos in just over two weeks". All the other forms I can think of take more words to qualify the "two weeks" to indicate a few days more than two weeks.
Using the text "in the last three weeks" means not more than three weeks and so matches the ideas wanted by the question.
Best Answer
I've heard 10 of 100 people used, particularly where it relates specifically to a survey of 100 people (effectively saying 10 people from a survey of 100), but it's unusual.
10 out of 100 people is the more commonly used option of the two.
Other options would include: