If the Norwegians mobilised on the 5th the Germans would most likely have failed. Just full readiness at all the forts would have sunk half the German navy or more. The Norwegians were very badly equipped, even worse trained and lacked all kinds of equipment, but the fighting around Narvik showed that when they had time to collect themselves, they fought bravely and learned very quickly. If the Germans are unable to secure the airfields (primarily Fornebu outside Oslo) undamaged, they will have to rely on long-range air support from Denmark and the odd planes they can place on hastily cleared ad-hoc fields.
The Norwegians could get 300 000 men in the field in case of a full mobilisation. One big what-if is the position of Sweden. If a majority of Norway bravely fights and hold for a month, I suspect the opinion will go the way it did with Finland - intervention, intervention, intervention NOW! From the start of the winter war, most Swedes expected the Finns to fold quickly, it was not until about a month into the conflict, when it was clear that the Finns were fighting like mad and actually holding the Soviets that the Swedish opinion changed and deliveries of materiel picked up to a torrent.
The Swedes and Norwegians use the same riflea and MG ammunition (6,5x55, a remnant of the Union dissolved in 1905) and the same light artillery ammunition, so Swedish supply could suite the Norwegians fine.
Mid-april, the Swedes had 400 000 men in the field and felt reasonably safe, the Finns were returning much materiel lent during the winter war and production was ramping up. I can see the Swedes fulfilling the pre-war orders for modern medium howitzers and Bofors 40mm AA guns. The Norwegians helpdes the Finns quite a bit with volunteers and materiel and I can see it being returned if the Norwegians fight on. A joint Swedo-Finnish volunteer Brigade is quite possible, as is transit instead of internment of the Norwegian 1st Division if it gets pushed across the border as in OTL.
The Germans may take Olso and perhaps Bergen, but not quickly and not without the Norwegian securing the important resources (weapons depots, mobilisation of men, the government, king, and gold reserve etc). Without airfields in Norway, they are going no-where fast against 100 000 angry Norwegians, who are among the best rifle shots in the world - harassing rifle fire did not exist in the Nordic countries, it was effective long-range fire, taking out officers and NCOs at an alarming rate for the Germans. The Norwegians said "skyt ham med tunnne ben" (shoot him with the tin legs, referring to the tight riding boots many German officers wore).
Add Swedish and allied deliveries of weapons and the Germans are in for a world of hurt. They do not have the resources to conquer a prepared Norway without seriously endangering the attack in the west.