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yes, bot of those situations work, as long as the assassin is untapped at the beginning.
the way it works is as follows: the decoy/sorcerer taps and its ability is put on the stack. then you tap your assassin and put its ability on the stack, on top of the first ability. last-in first-out then applies for the stack, allowing your assassin to sneak an attack in. this sucks for the decoy, but the sorcerer's ability will still resolve afterwards and your assassin won't be very happy.

He got it right, but as I pointed out it your other thread, rules questions should really go here, to avoid clutter.

Incidentally, the decoy's ability still resolves, I believe(unless it specifies target untapped creature), but tapping a tapped creature won't do much.

ya as long as its in efect of the attacking player declaring the attackers;
and all of these questions so far can be resolved just by remembering "filo"
First
In
Last
Out

first
in
last
out

and

last
in
first
out

that is the same thing

it really doesnt make much of a difference