Why is it not, 'she finds....'?
Sure! Why not?
Should the following also be accepted "she thinks that the collar of her dress is too tight".
Why is it that the "que" is elided in the translating?
Yours is correct and should be accepted.
"Trouve" is "finds," not "thinks." It should be "she finds the collar of the dress is too tight." This is a poor translation.
same question as Neil
She thinks that the collar of her dress is too tight was marked incorrect. Should be accepted.
And, too perhaps "She is finding that the collar..." could be an scceptable translation, as well.