Assuming her ability will not change, i've got some questions about her:

1. Can you place weaknesses and encounter cards beneath her that stay in your hand otherwise?

2. If you can and choose a weakness, will it stay there permanently as weaknesses cannot be discarded?

3. If you have no cards in hand at the start of your turn and a game effect reacts to drawing one with discard due to her forced abililty like oppresive mists or Armitage, can you end up without a card beneath her?

1. The rules say that you "may not optionally choose to discard a weakness". The effect on Amanda forces you to "[c]hoose a card from your hand and place it beneath her".

I would argue according to the letter of the law that discarding and placing beneath sb. is not the same, and therefore it is possible. (I may overlook another rule.) But in my opinion that would go against the spirit of the law.

2. Weaknesses can be discarded from your hand (like with Terror from Beyond), you just cannot "choose to discard" a weakness. The Forced effect doesn't let you choose to discard the card. Therefore, if you can place a weakness beneath her, you are forced to discard it.

3. I would think so.

Based on what we know right now, I believe that @Astrophil84 is completely correct. The rules as they stand them do not prevent Amanda placing weakness or hidden cards beneath herself, and from there they would be discarded since the discard is not a choice and does not occur from hand, getting around the specific wording of the rules for weaknesses and Hidden cards. The only counterargument within the rules currently is that Hidden cards are still encounter cards and therefore are owned and controlled by the scenario, not Amanda, so you could argue that they cannot be chosen, but the wording of Amanda's ability isn't paying a cost so I think that argument falls flat - and even if it doesn't, she'd still be able to get around player-type weaknesses like The Tower XVI and Dark Pact.

All that said, I am very confident that the rules will ultimately not support this - whether by means of additional rules clarification in the Innsmouth Conspiracy campaign book, or through developer clarifications and/or errata/FAQ to invalidate these interpretations. I certainly intend to treat her ability as incompatible with weakness and hidden cards, unless an official ruling comes down otherwise..

I'm 99% sure that weaknesses like Dark Pact and The Tower XVI are OK to put under Amanda Sharpe. It's very possible that her Signature Weakness will be such a card as well.

I doubt that Hidden cards can be placed beneath her.