Is a Ramadan compensatory fast valid if the intention was made at night, then the fasting person hesitated before dawn due to the unavailability of suhoor? And is there a sin or expiation for delaying Ramadan compensatory fasting without an excuse until time becomes short?

Hesitation in the intention to fast at night invalidates it, unlike hesitation during the day after the intention has been formed. Therefore, the fast mentioned is not valid as a make-up (qada'), even if you observed it as a supererogatory (nafl) fast.

There is no sin in delaying the make-up until the end of Sha'ban if there is hope of completing it before Ramadan. However, delaying the make-up without a valid excuse until the start of the next Ramadan necessitates both making up the fast and paying an expiation (fidyah) according to the majority of scholars, unless the delay was due to ignorance of its prohibition.