Your Shadow cards are not active on your Fellowship turn, so you can't stack Shadow spells. Therefore parts 1 and 2 are both "no".
The bonus question is interesting. I'd say you can. Opponents' Shadow cards are legit. Oh the possibilities :-)
This is a common misconception, and not quite right. Dwarves stack best, of course, and have some good examples.
Preparations, for example, can stack
any card from discard, and may be used with
Rest by Blind Night to shuffle a considerable number of Shadow cards back into your draw deck.
Ever My Heart Rises stacks anything from the draw deck, and
Hall of Our Fathers stacks anything from hand onto it, and takes anything stacked onto it into hand.
The rules on inactive cards are actually much less prohibitive:
active
During your turn, only these cards in play are active:
• sites on the adventure path,
• sites in any player's support area,
• your Free Peoples cards,
• your copy of The One Ring, and
• your opponents' Shadow cards.
All other cards in play are inactive. Inactive cards are not affected by the game and do not affect the game.
Note: all other cards
in play are inactive. As an aside, I don't think cards in hand are ever active or inactive - they're simply in hand. Try to refrain from quoting me on this, as I haven't really considered all that this means.
There's also some relevant information for stacking cards.
stack
Stacking a card is not playing a card. Stacked cards are placed face up and may be looked at by any player at any time.
Stacked cards are not in play and are not active. You cannot spot them. They do not count for uniqueness. A stacked unique card may be in play elsewhere. Multiple copies of the same unique card may be stacked together.
For what it's worth, stacked cards are not inactive - otherwise cards like
Gorgoroth Pillager and
Sindri would have no point. I imagine how the game handles cards stacked and cards in hand are somewhat similar - not in play and not active. We have plenty of rules on when you can play things from hand, but no rule allowing you to play things stacked unless another card gives you that rule.
So there's absolutely no reason you can't stack a card as long as it meets the requirements: none for
Preparations, Free Peoples item for Gandalf's Card, and a spell for
The Art of Gandalf. (1) checks out - legal.
(2) is fine, as well.
Hall of Our Fathers takes any/all cards stacked into hand, and if it weren't legal there would be no need to specify Free Peoples on
Dwarven Foresight. Most other cards do specify, but the ones that don't aren't held accountable because of them. Again, stacked cards are only as valuable as the text that tells you what to do with them.
The Art of Gandalf is a different card from others, and has a different functionality.
Which means that (3) must be legal. There are no other ways (I can think of) to take an opponent's card into hand, but that doesn't mean it's against the rules.