Family union visa or Tourist visa?

1. Is it possible to bring her on short term stay visa – C and get it converted to long term visa while staying in Belgium?
-> NO, not anymore.
2. Is it true that once she gets a short term stay visa – C, she cannot apply for long term visa for next 1 year?
-> I don't think this is true. You may call VFS for the official answer.
3. What is the general waiting time to get short term stay visa – C?
-> Short term visa is normally approved quite fast if the documentation is perfect. It is issued in of 3-4 working days.
4. Do we have to apply for Schengen visa separately or a short term stay visa – C to Belgium will be by default a Schengen Visa?
-> Is it the same. Short term visas are always issued as schengen visas unless Embassy has a particular reason to not do so.
5. What would be the starting date for short term stay visa – C; will it be the date I mention on the application or the date when visa is granted? Will the Visa be issued for 6 months and then she can stay for 90 days?
-> Short term visas is issued for max duration of 89 days. *Normally* the start date is the date which you mention on the application as "expected date of entry", and from then, the number of days maximum of which is 89.

Thank you