1023456789
It is difficult to interpret exactly what you are trying to ask. If you allow your five digit number to have leading zeroes and do not allow negative numbers, then 00000 - 99999 = -99999 is the smallest number. If you do allow negative 5 digit numbers, then (-99999) - 99999 = -199998 is the smallest number.
9
It depends on what is meant by "using" - in a string or with operators; and are leading zeros allowed? The smallest positive string, with no leading zeros, is 1023456789. If a leading zero is permitted, then it is 0123456789. Once you allow operators, it depends on which operators, and the question becomes extremely complex.
The smallest number that someone can get using the 91764 digits is 14679. The secret is to arrange the digits from the least number to their greatest number.
9
The smallest even number using this combination of digits is 2568.
The smallest value Roman numeral using each symbol only once would be MCDXLIV = 1444
01489
1368
19
Largest: 76542 Smallest: 24567
7, 14, 28, 56, 903