YOU SAID:

Challenges remain as to whether that

INTO JAPANESE

それが可能かどうかについては課題が残っている

BACK INTO ENGLISH

Questions remain as to whether it is possible

INTO JAPANESE

それが可能かどうかについては疑問が残る

BACK INTO ENGLISH

I doubt it's possible

INTO JAPANESE

可能かどうかは疑わしい

BACK INTO ENGLISH

I doubt it's possible