The moment the Germans attack Norway, Norway joins the Allies. So, if the Germans land their paratroopers in Norway on the '0' chit (which they do not have to as planes can move on any chit draw), they are at war with Norway and Norway is an Allied power as of the German 0 chit draw.
If the British drew their '0' chit on the next draw, they could reinforce the battle that is already taking place (the only at start units in the battle and eligible to be front line for the first round of battle would be the Norwegian 2-3 infantry unit and the German 3-5 para unit). Everything else that moves in (i.e. the other Norwegian infantry and any potential British landed via transports) would be placed in reserve and could potentially join the battle on subsequent rounds as reinforcements. The same would be true for the Germans, if they decide to move a transport with 3 infantry (along with a naval leader since the 0 has already been drawn) on a later chit draw, the additional German infantry would be in reserve for the first round, but could potentially move up as a reinforcement for later rounds.
Hope this helps.
Mark