1. Does reloading during working hours present any risks from the Qlikview side or the source database (DB2 in my case) side, such as data corruption, inconsistency or conflicts?
A reload means retrieving data from the source DB. That may have a negative impact on the performance of the source database and the machine executing the reload. If a qvd is locked by a process it cannot be written to and a reload that needs a locked qvd will fail. Make sure that the task that reloads a qvd finishes before another reload uses that qvd as source. Data corruption is extremely unlikely unless you have flaky hardware. A reload can't cause corruption in the source database unless you're doing very silly things that involve writing to the database.
2. How often can such QVD generators be reloaded, is there any limits?
If a reload takes 30 minutes then you can reload 48 times a day if the source database is online, the machine that executes the reload is online and there's enough cpu, ram and harddisk capacity available to successfully reload.
3. What is the best practice to offer users with dashboards that are updated more often than 24 hours?
Reload more often.