Yes, you'll need "quests_reaction.c" and "both_reaction.c". However, "both_reaction.c" does not need to contain much, it just needs to exist. Have a look at the one for the "Hornblower" storyline, it's a minimal file which does nothing.
One thing to note: you'll also want to create the folder "RESOURCE\INI\TEXTS\ENGLISH\Storyline\Witcher". You can see the sort of things which go into the similar folders for other storylines, including the storyline's "characters_names.txt" file. As of next update, these "characters_names.txt" files are going to contain
all names from their respective storylines (and "RESOURCE\INI\TEXTS\ENGLISH\characters_names.txt" will contain all names from the general game). Names aren't normally translated, but we have someone working on a Russian translation and Russian uses a completely different alphabet. So please create the file for your storyline and include the names of the entire storyline cast.
Don't copy "Tales of a Sea Hawk"! Have a look at "Hornblower" instead, it's much easier to follow because it's a very linear story - pretty much everything happens in sequence and "quests_reaction.c" follows that sequence. "Tales of a Sea Hawk" is a horrible mess - if anything, it's a good example of how not to write a storyline.
Or have a look at that thread about my "Ardent" storyline and maybe download the earliest version, which was also fairly linear as it only got as far as the escape from prison. Once you've got the story started, you can build it up gradually - play it through, check that it works, figure out why it doesn't work, fix it, then add the next bit and repeat.