In what situations or contexts is 'seten' used to convey 'some' or 'few' as opposed to 'kek'? Is it used in the same way as saying ' there are certain people...'?

Yes it is.

Sèten → some
Kèk → some, a few
sèten or kèk  is used to translate some


Gen sèten jou mwen pa fouti leve nan kabann.
Gen kèk jou mwen pa fouti leve nan kabann. (is also correct)
There are some days I can't get out of bed.

Gen sèten bagay mwen pa ka di w kounye a.
Gen kèk bagay mwen pa ka di w kounye a. (is also correct)
There are some things I can't tell you right now.


Sèten also means sure, conclusive, clear

Mwen sèten w'ap fè bon desizyon an..
I'm sure you'll make the right decision.

Mwen pa twò sèten si m'ap kapab vini aswè a.
I'm not sure I can come tonight.

Ask me anything