I have often wondered the very same thing. While I cannot give official advise, I think some sound judgement may dictate what to do. For example, fire ban when you enter and you ahve that huge rain storm. After a day is the earth still moist? What about the underbrush and downed limbs, trees, etc.? If you believe there isn't a risk of forest fire then I think a campfire is ok. Same goes for the second scenario you mentioned. If when you enter there is not a fire ban, but a ban is put into effect whilst you are enjoying the park, use some common sense. If its windy and dry, probably not the best idea to have a fire.

I think also if you have a fire and aren't allowed one and a ranger comes your way they could explain to you that a ban has been put into affect and check your permit to see when you entered, realizing there wasn't a ban at the time.

Basically, think.

I believe that common sense should be your guide.
One year I picked up my permit in Atikokan and they had a fire ban on even though it had been raining fairly heavy and the forecast was for more rain the next few days.

I asked the lady whether or not the ban would be lifted in the next few days?
She said that it wasn't their call but that obviously the fire danger was greatly diminished and I should use my best judgment.

I said that with the cooler temps forecast for start of trip that a nice warm campfire was something I looked would welcome after a long day on the water.

She smiled and said they wouldn't want anyone suffering hypothermia or being miserable when the conditions had obviously changed but the ban hadn't been officially lifted yet.

When I hit Beaverhouse, it was raining buckets and at the portage to Quetico lk, I ran into a Father and Daughter tandem who said the last few days had been miserable with the cold/wind and rain, but they were afraid to build a fire becasue when they went in a fire ban was on.

I told them it still was and what the ranger gal told me and that I planned a fire that night, ban or no ban.
The Father looked a little sheepish and the daughter said something like " I told you they wouldn't care, it's been raining cats/dogs for the last 3 days.

I can't imagine a back country ranger going without a fire when the conditions changed before the ban could be officially lifted, and doubt very much they would expect you to do the same.

A long time ago we went in from Beaverhouse when there was a fire ban.  You could use a gas stove but no fires.  It was great no bugs and the sun shined every day.  A week plus later we were paddling out on Quetico Lake and had stopped on a sand point.  As we were walking on the point, a plane landed and the ranger basically asked what the hell we were doing in the park as the park was closed.  We showed him our permit with our entry date so he didn't do anything, but he advised that we had to leave the park right away.  So if it gets dry enough, they will come and kick you out.
Westwood

Based on what I have read and discussed with rangers, I believe what snow dog said to be the case.

Ploder,

A few years ago I was soloing up on Quetico lake. I tested my gear before I left, but my first night up there my stove wouldn't work (the gasket on the pump and the valve had failed so no pressure would build and any pressure that DID build simply pushed fuel out through the control valve). I was fine cooking on a fire for the during of my trip but that got changed when I had three days of soaking rain. I couldn't get anything to catch fire it was so wet. I had to turn back because of it.