If I have evolved a Pokemon by placing a Stage 1 or Stage 2 card on top of it, can I play attacks from the basic form?
There are a few ways to do that, but not normally. The easiest in standard is Memory Energy:
This card provides Colorless Energy. The Pokémon this card is attached to can use any attack from its previous Evolutions. (You still need the necessary Energy to use each attack.)
Also, BREAK Pokémon do still have the attacks of their last evolution (so Raichu BREAK can use all of the attacks of whichever Raichu it evolved from, in addition to Grand Bolt).
However, the norm is that no, you may only use the attacks printed on the current Pokémon's card.
No, you can't. A Pokemon's available attacks are only those printed on the card; those in its basic form are effectively "forgotten" and cannot be used (unless the Pokemon devolves somehow).