teacake, I understand exactly where you are coming from.
Three of my neighbours were given the Pfizer jab followed by the second one after 3 weeks.
Next neighbour to have it was told it would be 12 weeks before he gets the second one, and this is because the government changed it to this.
As you have rightly said, Pfizer have not said this will be ok to wait 12 weeks, and this is why you are concerned.
I share your concerns, but after giving it a lot of thought, I've decided to go ahead and have my first jab this coming Saturday simply because I do believe it will give me some protection and if I have to wait 12 weeks for the next one, then so be it.
There is always the possibility that it could change again and we may not have to wait as long. I hope you decide to go for it because as others have said, even one jab should protect you from having to go into ICU if you were to get it. Good luck.