Is is Ok to use "denojou"(french, De nos jours) alongside the others, "sejousi", "sètansi"? How would one say "in the olden days" with one word instead of a phrase like its opposite above? More Synonyms for both(if possible)?

Yes,

these days 
denojou
sèjousi 
sètansi
sètanprezan
aprezan
alèkile
alòkile
kounye a 
kounye a la

In the olden days (not all the followings are one-word, but Haitians have a lot of  "expressions" for the olden days
lontan (long time ago)
otrefwa (a very long time ago)
tan dantan (good old days, olden days)
tan lontan (in the old days)
tan benmbo (a very very long time ago)
depi digdantan ( a very very long time ago)
tan prezidan bann machwè ( a very long time ago)
depi ti konkonm t'ap goumen ak berejèn ( a very distant past)

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words

Tan dantan?

Tan dantan
long time ago.
the good old days
olden days

chante tan dantan yo
the songs of yesteryear


Ayiti dantan
Haiti in the good old days