The problem comes with the fact that, other than a first adventure, a GM has no way of guaranteeing the specific XP starting point of players, yes?
Not true. When you kick it off, you know exactly how much XP a character has in the beginning. It is only math to determine where they will end up at the end, before applying TBX.
It's a DM's responsibility to recognize that characters may level and plan or adjust their adventure accordingly.
Yes, a good DM will take this into account.
But, you just hit upon the crux of ruling. It is the DM's prerogative with Judge approval. And guess what, those leveling up issues are posted in the DM's rules for the adventure ahead of time. You don't like what is published ahead of time, don't play it.
That inconsistency is unfair to players. One GM allows characters to level as soon as encounter & time xp gives them enough xp to do so; another allows leveling but only under certain conditions; and still another doesn't allow it until the end of the adventure.
What is "unfair" by not getting to level-up in the middle of a dungeon? Your level in not a way to keep score, is it? Can you prove to me that you are losing XP by not getting level-up or having to wait until adventure end to receive your TBX?
The encounters are set for a certain APL, if you raise the APL then you have to raise the CR. If you don't raise the APL, then the CR is fine. If you have to play at a certain level, not getting to apply your TBX, are you getting cheated? No, you are not, you get all your XP in the end.
You guys sound like greedy XP hounds and want the DMs to work harder. DMing is a lot of work, you better recognize that before you start telling them they need to do more.
I have a counter point to GE's overall complaint about long adventures. Dagger's at Midnight is a short 4 encounter adventure that is dragged out by lack of decent participation and slow posting rates of the participants. For months the characters have done little to advance the game (some posting once every 2 weeks or longer), yet earn the same XP over time. So while they screw off and essentially earn an extra level by doing nothing, I have to modify my adventure to accommodate it? Nuts to that. You start dictating that sort of thing to me, I will shut the game down first.
Now, Scourge of the Howling Hordes is a great example. Due to a heavy reward for having encounters stacked on each other, the time based awards could have the players advance again after only one encounter since leveling the first time. This is a bit ridiculous to me as this is not a heavy role-playing adventure but a dungeon crawl and we update practically daily under normal circumstances. The characters still haven't rested since leveling up the first time yet. However, I think things will work out. They should realize that they don't have 3rd level gear yet so the longer they stay at 2nd level the better off they are for balance reasons.
DC's adventure is balanced for their characters to complete without leveling up. It was not advertized to allow leveling up nor mid-adventure shopping. Whining about not getting to level up mid-adventure is essentially saying you think your character should gain advantage for the final encounters instead of playing as it was designed. It is a short adventure that is in no danger of dragging out into heavy role-playing without rewards. I stand by my Judgement on this when I approved it.
Another argument against repeated mid-adventure leveling up is Crypt of the Everflame. We are advancing from 1st to 4th level, yet still fighting in first level gear. I find that pretty frustrating myself, and Tyrien has some 2nd level gear. So to keep things balanced to our level, we are fighting higher CR monsters, yet do not have gear appropriate to our 3rd and 4th levels. Our current fight has every monster with an AC higher than our best person. I like having characters advance, sure, but I also like them to have the full perks of being that level.
I think the pace we should be setting for leveling up is about 1 level every 2-3 months and about 3-5 encounters. Leveling up after 1-2 encounters is a bit ridiculous. This is part of the Judge's responsibility when approving the DM's adventure.
Bottom line, Judges jobs are to keep things fair. There is nothing unfair about not getting to level-up mid-adventure or having to wait until the end before getting to apply your TBX/TBG.