*How serious is saying "Je t'aime"?*

Serious.

*Did the Serge Gainsbourg song Je t'aime…moi non plus change the meaning of the phrase je t'aime?*

No, *je t'aime* has the same meaning it still has today in that song.

*If I recall correctly, before the Jane Birkin version was released in 1969, it was not a strong statement, and meant more like Je t'aime bien does today.*

No, it was already a strong statement in 1969, as it has been for many centuries.

*Back in those distant days it took some flowers to get the message across.*

Flowers could complement or substitute the words, but were not obligatory.