Does a countered prepare spell stay prepared?
No. Casting the waiting spell copy makes its permanent unprepared, even if that spell is later countered. A prepared creature changing controllers lets its new controller cast the copy. Merely copying a creature does not copy its prepared status.
For example, do not leave a prepared marker in place just because your opponent countered the spell. Track the act of casting separately from the spell's eventual result. See the mechanics walkthrough for the basic sequence.
Can empower Jace use a normal Jace card?
The instruction checks for a Jace planeswalker token, not any Jace permanent. If there is no qualifying token, create one before adding the stated loyalty. If a spell fails to resolve because all its targets are illegal, its empower instruction does not happen.
Before resolving a spell, identify its targets and the qualifying token separately. A familiar Jace card sitting nearby is not a substitute for checking what the instruction actually requires.
What changed about attacking a player alone?
The September 21 rules bulletin clarifies that an effect looking for a creature attacking a particular player alone counts the creatures declared attacking that player. Attackers aimed at other players, planeswalkers or battles do not count against it.
In a multiplayer combat, group attackers by their actual destination before checking the condition. Read the precise ability: this is not a blanket replacement for every use of the words 'attacks alone'.
FRA is not FRC or Special Guests
A booster product name does not by itself tell you whether a card is legal in your format. Check its set code, the format's card pool and any applicable restrictions. Use the official release notes for FRA, FRC and Special Guests distinctions rather than assuming every card opened in a Reality Fracture product is Standard-legal.
The explorer deliberately labels its FRA scope. The Commander decklist guide is separate from the Limited tools.