Per your question, the header and studs below the window can easily hold the 75# weight.
For a much cleaner look, I would look into a ceiling tv mount. With the ceiling mount, you can rotate and tilt the TV. Very clean looking. TV looks like it is floating. This is a 70" tv, also in the 75-100# range. Attached are two pictures of the TV w ceiling mount.
Yes it will not be an issue. The header is almost surely over-sized as it is not designed to just barely support the design weight but rather it is up-sized to the next sized header that is being used on the project. Even if it was a max capacity, 75 lbs is relatively small compared to the amount of load that the header is designed for.