"...the government steps in to investigate and finds them guilty and legally shuts them down permanently, will there be tons of suicides?"
Perhaps a few but not that many. Power hungry elders will probably step in to fill the void. The prospect of becoming a GB member or one of the top dogs will be a lure for them.
During Rutherford's reign up to 3/4 of the witnesses left. It was still able to grow again and bigger than before.
However, what may be interesting to see is what would happen if the GB were to abandon the Organization with tens of millions of dollars in their pockets. Even then the Witnesses will think that the forces of evil killed them or that they are hiding from persecution, ready to communicate with them in secret, since the end is near.
Also, it is very doubtful that government has the power or inclination to shut down any religion.