It is true that for income tax purpose you can only use the declining balance, it is a tax rule. It is a good choice to decide on the declining balance for accounting purpose in Quickbooks, for consistency.
To answer your first question about the remaining value left at the end, you need to continue the depreciation, and when you reach the last year, there will be a negative amount. As we cannot have a negative value, the software will change this to zero 0. With the straight line you are suppose to be using a salvage value.
This is the link to the guide for business tax return, which also include CCA:
http://www.cra-arc.gc.ca/E/pub/tg/t4002/t4002-13e.pdf It is from Canada Revenue.
To answer your second question, both, depreciation and CCA, approach are very different and the number of years could very well vary. With the straight line method you expense the same amount every year, and it is going to zero value much faster. With the declining balance, it is 55% of the value that is left, therefore it is never the same amount that you expense every year.
Example: with declining balance, you expense your car that you use for business, the rate is 30%, on tax return. You bought your car $10,000; calculation for first year 10,000 x 30% x 1/2 (half year rule) = $1,500. The value of your car is $10,000 minus 1,500= $8,500. Second year: undepreciated value: $8,500 x 30% = $2,550. Value year 2 $8,500 - 2,550. You see that it can take more time with declining balance.
