at all; and that

Question

Harry had never even imagined such a strange and splendid place. It was lit by thousands and thousands of candles that were floating in midair over four long tables, where the rest of the students were sitting. These tables were laid with glittering golden plates and goblets. At the top of the hall was another long table where the teachers were sitting. Professor McGonagall led the first years up here, so that they came to a halt in a line facing the other students, with the teachers behind them. The hundreds of faces staring at them looked like pale lanterns in the flickering candlelight. Dotted here and there among the students, the ghosts shone misty silver. Mainly to avoid all the staring eyes, Harry looked upward and saw a velvety black ceiling dotted with stars. He heard
    Hermione whisper, "It's bewitched to look like the sky outside. I read about it in Hogwarts, A History."
    It was hard to believe there was a ceiling there at all, and that the Great Hall didn't simply open on to the heavens.
(Harry Potter and the Sorcerer's Stone)

It is hard to understand what this phrase, ‘and that’, means. That is because, I think, the two intensifiers, ‘at all’ and ‘simply’, have more than a couple of meanings - in vain I racked my neurons over several my mother tongue’s words. And that’s because of this phrase,‘and that’, that I do not know how to interpret.

At first, I thought ‘believe’ takes two objects: (object 1) there was a ceiling there at all, (object 2) that the Great Hall didn't simply open on to the heavens. It seems to make sense at sometime, when aforementioned two intensifiers are mingled seemingly harmonious with ‘and that’ somehow, but before I knew it wouldn’t make sense because the twos change their meaning clothes second by second. Once ‘and that’ leads resulative adjunct, next ‘and that’ turns into objective adjunct, next exclamatory adjunct.

To solve this problem, I’m in need of understanding the meaning of ‘at all’ and ‘and that’. I habitually thought that ‘at all’ intensifies the fact that he can’t believe, but I just before read in Webster’s Learners that ‘at all’ can intensify not only negative statements but also statements. This makes me to read ‘and that’ as a resultative adjunct.

How do I understand those three: at all, and that, simply; and finally the sentence whole?

Answer 1

Historically at all meant altogether, entirely, in every way, but since the 17th century it has been used only as a ‘negative polarity item’: an expression which occurs only in a negative environment. It is licensed here by hard, which is a ‘negative trigger’ like hardly, barely, few. John Lawler's answer here gives a brief introduction to NPIs, with more links.

Consequently, at all now means (not) in any way or (not) in any respect:

There is hardly anything there at all.
There is nothing there at all.
If he was at Harvard at all, it could only have been for a couple of weeks.

At all is thus an intensifier, as you suspected: it strengthens the negativity of hard to believe by characterizing the apparent absence of the ceiling as virtually complete.

As wmassingham says, and that introduces a conjoined second DO of believe. What is interesting here is that the conjunction does not really represent two different things that Harry found hard to believe, but two different representations of the same thing: the second clause recasts the negative presence of a ceiling as the positive presence of an opening!

Answer 2

It's a way of being perfectly grammatically correct and complete.

It was hard to believe there was a ceiling there at all, and that the Great Hall didn't simply open on to the heavens.

This should be broken out into two complete thoughts:

It was hard to believe there was a ceiling there at all.

and

It was hard to believe that the Great Hall didn't simply open on to the heavens.

"And that" is not a unit by itself; the "that" belongs to the part about the Great Hall.

(Unfortunately I'm not up on all the technical grammar jargon, so please forgive me if my terminology is not completely specific or accurate.)