White's non-e4 opening, do they all need a solid understanding of 1.d4 first? I'm talking about the two major alternatives apart from e4/d4, ie c4 and Nf3. It seems to me that a lot of the lines will transpose to a d4 type. Sometimes white plays d4 and transpose. Sometimes black plays d5 and white goes into a Queen's pawn defense with an extra tempo.

So is it possible to learn any of these openings without a solid understanding of 1.d4 first? I feel insecure that black can always trick me into a Queen's pawn opening.

Also a related question. Is it possible to learn the English 1.c4 without first playing Sicilian as black myself?
There are c4/Nf3 lines where White never plays d4 (such as the reverse Dragon or the King's Indian Attack). They are solid but not top-tier openings. Additionally, they do not feature much diversity of pawn structures, which can impede learning.

Specifically after 1. c4 e5, this is less relevant because the game does not usually return to 1. d4 structures.

My advice to a player that is trying to improve would be to play 1. d4 for a while first, or learn enough of the d4 transpositions that there is no fear of being tricked.

If not trying to improve, then almost anything is a reasonable choice.