Here is what you will need to free the stem.

Automatic Transmission Fluid
Acetone
old bicycle tube or other smooth rubber scrap
2x4 pieces, 4" and 24"
old cookie sheet or similar
Hammer

Place the cookie sheet on a concrete floor
Place a 4" piece of 2x4 in the center of the sheet
Place the rubber separator on top of the 2x4
Very lightly snug down the stem bolt
Invert the bike and place the head of the stem bolt on the rubber separator, support the bike so it is stable and the steerer tube is vertical.
Make a 50-50 mix of acetone and ATF and mix well
Put approximately 1 shot glass worth of the mix in the bottom of the steerer tube


If the mix can in any way reach the handlebars you may need to remove/unwrap them first or the Wrap may get damaged.

Give this several days before you stand the 24" piece of 2x4 to on a piece of rubber over base of the steerer tube and try to drive the tube down over the stem.

The Cookie sheet protects the floor from the mix, the rubber protects the finish on the stem from the 2x4 pieces.
Time is your friend here, if the mix runs out add more and give it a thump every now and then. You're not trying to sledgehammer it loose just a sharp blow to jar things.

That's my advice and it's worth what you've paid