Discuss all changes with the architect. What you think is minor may be major in reality.

The first shouldn't be a big deal and can be discussed with your GC.

I wouldn't do the second at all,

I’d check with your GC. They’ll help you check the gas fireplace specs for rough in dimensions (width, height, depth) to make sure it’ll fit on the wall you have in mind. They’ll also make sure the new location is feasible for venting and required clearances.

The answer is "It depends." If you're building in a community with architectural covenants and an architectural review process, changes to the exterior need to be reviewed by the architectural review committee or its equivalent.

The change in fireplace could have more than mechanical implications. For example, if you have a ceiling treatment (e.g., coffers) designed to coordinate with a corner fireplace, they may not work well with a fireplace that's not a corner fireplace.

A quick telephone call to your architect can prevent heartburn later.

If the local authority requires the permit application drawings to be revised or additional drawings submitted, the archtect would need to do that so call the local authority.

Mark:

My neighbor in Michigan built his house too close to the adjacent lot. Developer told him to move his house or buy the lot because the lot was now unbuildable. He bought the lot, tried to get a variance, and was turned down.

15 years later his house burned to the ground. He built it in the right place this time and the lot is again buildable.