Yes an NFL pitch is quite significantly narrower.... Take a look at some of the pictures from the Wembley games and you can see this. Also when the Seattle sounders play the fans are close to the pitch, but when the Seahawks play there is a lot of room between the stands and the pitch.
A possible solution would be for an astroturf pitch to be assembled over the top of the WHL turf, however due to all of the players and officials on the sidelines in NFL it would result in the first 4 or 5 rows of seats in the stand on that side pretty useless views when in NFL mode. A feasible solution would be to dig down several metres and have the WHL pitch on hydraulics, it could then be used at it's raised height for football matches and sink down with an astro pitch laid above for NFL, concerts, etc, etc. In NFL mode the astro pitch could operate at a level 2 or 3 metres below the normal level of the football playing surface, thus giving the spectators the right viewing experience even if in the first few rows. The question would be whether the astro pitch could be assembled/disassembled in only 1 day. There is also the question of how the weight of the astro pitch could be supported, but I would imagine it's not beyond the realms of possibility that huge steel supports could be housed under the North and South stands that can slide out to support the weight of the astro pitch.
Far more likely however is that the NFL just continue to use Wembley as it has been pretty successful there.