Oh ... there seems to be a guess already ... that's why even though I'm not done yet I'll share my work so far.
--- After edit ---
Now here's my solution
Horizontal:
D2-F2 The sum of three consecutive prime numbers
First there was a eight options but the Braille clue gave away that the number starts with 1 so the prime numbers left to consider were 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71.
They make up eight sums: 109, 121, 131, 143, 159, 173, 187, 199
Of these sums can be easily dismissed since they contain double numbers or zeroes leaving us with: 143, 159, 173, 187
The Orwell clue rules out the options containing 4 or 8. What's left: 159, 173
Further sudokuing (is that a word?) reveals a number 5 in the square ruling out 159 and leaving us with 173 as answer
B3-E3, F9-I9 A novel by George Orwell
Quick googling reveals 1984
A5-C5 Number of countries in the world
193 if we count UN-countries only, 195 if we count the other two, but the digits 19... are definite. After some consideration ... it kind of says all countries, therefore 195 must be the answer.
H5-I5 The number of minutes an iguana can stay under water
Googling says 30 but that cannot be true since 0 is not a number found in Sudokus, numbers higher than 30 don't make much sense when googling, so a leading 2... seems plausible but I left that open for now since that is not definite.
Since 26, 27 and 29 can be ruled out by sudokuing and the number has to be at least close to 30, the solution to this one must be 28
A6-B6 Bones in your foot
26 definite answer
B7-C7 Giraffe tongue in inches
Google says 18 - 20 inches, which means 18 or 19, since 20 contains a zero. However, the 747 clue gives away that it really must be 21 even though I didn't find that info during research.
B7-G7 Number of litres of fuel a Boeing 747 airliner can hold
I found online that there are several types of Boeing 747 which have different fuel capacities that begin with a leading 1 or 2. But the longest road clue gives away that it cannot be 1, so there is two types left. The numbers given by the cube of square clue and the octomino clue rule out the larger one leaving 216847 as definite result.
Vertical
B3-B6 Longest street in km (or the popular misconception for it)
Guiness World Records listed Yonge Street for some time but it only has 56km. However, a common misconception said that its length was 1896 km which must be the answer
D2-D5 Braille alphabet was invented
1824, easy, look it up
D6-D8 The number of octominoes
Definite mathematical thing -> definite answer: 369
C1-C3, G7-G9 A cube of a square
The only cube of a square with three digits: 729
H1-H2 A square
Possbile two digit squares: 16, 25, 36, 49, 64, 81
We can rule out 81 with the Orwille clue an 25 with the Iguana clue having now 16, 36, 49, 64 left as guesses.
Some further sudokuing and the 1 from the Ga clue rule out every number with a 1 and a 6: 16, 36, 64, conveniently leaving 49 as final answer.
H3-H4 Ga
This could be a reference to Gallium, which would be 31 on the periodic table.
This could also be numerically encoded, resulting in 71
Having only this two clues I dare make the conclusion, that the second digit is ...1.
> Having 576 as three digit square rules out 71 leaving 31 as answer.
I2-I4 A square
There are a total of 22 possible three digit squares but ruling them out with the rules of Sudoku left me with: 529, 576, 625 and 691
Getting 49 in the double digit square clue rules out 691, leaving 529, 576 and 625
Some sudokuing after guessing the country clue we get 6 as definite last digit and therefore 576 as answer.
After having solved all clues everything left was to solve the Sudoku which went astoundingly fast.
My solution looks as follows
--------+-------+--------
| 8 3 7 | 5 9 6 | 1 4 2 |
| 6 4 2 | 1 7 3 | 8 9 5 |
| 5 1 9 | 8 4 2 | 6 3 7 |
--------+-------+--------
| 7 8 3 | 2 5 9 | 4 1 6 |
| 1 9 5 | 4 6 7 | 3 2 8 |
| 2 6 4 | 3 1 8 | 5 7 9 |
--------+-------+--------
| 9 2 1 | 6 8 4 | 7 5 3 |
| 4 7 8 | 9 3 5 | 2 6 1 |
| 3 5 6 | 7 2 1 | 9 8 4 |
--------+-------+--------