Yes, "the blonde chick" is Samantha Carter. The actress playing her just had a baby, so she was off the show for six weeks.

"MacGyver," also known as Richard Dean Anderson, is basically retiring to spend more time with his daughter in Los Angeles. (They film the show in Vancouver.) He's been transitioning off the show since last season. Hence the bringing on of Ben Browder.

He will supposedly pop up every now and then.