Are you trying to gather information with which to present to an airline in an effort to gain compensation/refunds for delayed/cancelled service out of interest?
I honestly don't think you'll have any luck by suggested engineering was incompetent and should have found this fault long before it manifested itself as a leak.
These things happen all the time.
In answer to your questions;
1) Depending which pipe it was, hydraulic pipes in the pylon area can be an absolute nightmare to fit for access, as Fargoo says, taking many hours. Usually the biggest wait will be identification/awaiting receipt of spares though.
2) Yes, it is usual for items like hydraulic pipes to be shipped from Airbus. Most airlines for most parts will have local loan agreements with other carriers, meaning a lot of spares will be available at a lot of stations. But you can't cover everything (usually LRUs/valves/wheels etc). And certainly it would be rare to find hydraulic pipes!
3) Common - that depends. An airline may have one occourance a year of a hydraulic leak from that pipe. Airbus may be aware of more instances, and if they do become aware of issues like Fargoo mentioned, a SIL will usually follow.
4) Yes, it is possible, but you'll be lucky to find someone who will risk their job to tell you, especially if its part of a claim against the airline they work for!