Of Course you can shut the main valve and NO IT WILL NOT LEAK if the valve is in decent condition.
I have shut water main valves over 100 years old and not a problem
Before you take any plumbing advice as gospel consider the source as a Master plumber normally gives their name and ways t contact them
If your in a colder climate just make sure you have enough water for your heating system such as a steam boiler.
Shutting off the mail will save you not only a large water bill it will save you a fortune in water damage
RE: your water heater NO it will not mess it up but you will not have no hot water if the main is shut
Also as long as the water heater is below most of the facets in your home there is little danger of siphoning the water out.
You may want to lower the temperature on the heater when not in use
I do not understand if repairs were made other then a possible freeze up why you would have another severe leak?
Was the plumber licensed? What was the cause of the last leak?