Hi again Anna.
First of all if you are based in the UK then these two situations need to be examined under the fair dealing exceptions, not fair use which is an American doctrine. Broadly speaking anything which would meet the fair dealing exceptions would also be fair use, but not vice versa.
So that out of way, let's look at your two situations. The first thing to note is that each of your videos would be treated as a separate work and so whether or not it might infringe another work ('the documentary') will be limited to the quality and amount of the documentary that is used in each individual video on its own.
Situation 1. Assuming that the small parts of the documentaries / news programs are as brief as is necessary to make the point you wish to make, then this should be fair dealing, either as quotation or as review/criticism, so long as you credit the source material. There is nothing in theory to stop you repeating the same small extract several times within the same video, as the test is whether a substantial part of the original
has been used, not how substantial a part the extract plays within the second work (ie your video). Of course I am just looking at the possibility of infringement, not whether new copyright will exist in your video. A so-called derivative work needs to contain sufficient original material to reflect the spirit or personal creativity of the author of the new work (you) in order to gain copyright for itself. Clearly if the derivative work is entirely made up of snatches of other copyright works, then whether it meets the creative criterion would depend on the selection and placement of the edits, which is a much harder characteristic to judge.
Situation 2. Again, provided that you quote your sources and each of the large number of separate extracts is the minimum you need to make your point, that would be fair dealing assuming that you didn't effectively copy the whole of the source work. Having met that test successfully, you can repeat the same extracts in as many separate videos as you like.
If you have time, here
is a link to a recent case in the High Court of England and Wales which examined something very similar to your situation one scenario. It concerns a documentary about the first Beatles tour of the USA, which drew heavily on contemporary footage of one particular concert. In fact it used virtually the entirety of the footage of the actual songs filmed in the original, albeit in small chunks, and this was found to be infringement of copyright in the songs, since it was done without permission and to a degree which fell outside either fair dealing or fair use. The case provides an insight into how the courts go about deciding cases like this. Unfortunately the judgment also contains a lot of other detail which has no relevance to your question, so you may need to skim over those parts,