If You Fail The Roll, Can You Use A Clue Token To Reroll?

No, because that roll is not a skill check. You may only spend clues to add dice to rolls that are skill checks.

And, in case you missed it (as we all have), the roll for gaining brood always occurs, even before final combat. This text just happens to be located in a weird place on his sheet.

And how painful that roll during the game can be. From a recent game: Akachi had already sealed one gate and had entered a gate at Hib's to get her third gate trophy to pass her personal story. While in the OW, a surge occurs placing a cultist on the gate at Hibs. Then she gets an encounter that sends her back early, so the next turn she has to evade or kill the cultist before she can close the gate. She goes for the kill (figuring it's only going to be her second brood token, so she's got a five in six chance of living). She succeeds in killing the cultist, gets her brood token, rolls the die ... and gets a 1.

In the end , the investigators got the last laugh, managing to defeat Eihort in final combat mainly due to Harvey having passed his personal story and having Voice of Ra and two Shrivellings.

Up until recently we played Eihort wrong. I thought that his Brood ability is only active during final battle.(due to its irregular position on the sheet).