JD,
Progress is a challenge you seem to be meeting.
The grounding of posts in a garage is fraught with two problems. One is bumping into them with the vehicles. A very secure fastening is called for. The second is water, The bottom of the post wicking up water is a fast route to failure. You can lick both devils with a metal pan post bottom which is secured to the concrete and to the post. If it sits slightly off the floor so much the better. There are commercial post bottom pans available or you can fabricate your own. A strap up from the pan on the unseen side of the post is easily lag screwed to the post. The pan can be gunned to the concrete or use a jbolt when you pour. By all means slobber the post end with sealer and use a sheet (cut to size) of ice dam rubber between the post and the pan. Moisture gone forever.
It sounds like the header extensions beyond the outside posts lend themselves to braces back to the posts. A 2.5 foot long brace made of stock half the cross section of the post (or slightly smaller) & header should work fine. Cut the tenons on the end of the braces to insert into the header/posts at right angles even though the tenon will be triangular in basic shape. Inlet the mortices about 1/2" so the brace goes in first is is very secure. A Single peg for insurance after the structure is up.
There many technques to attach the stick frame to the post and beam elements. Many old barns cut shallow open mortices on the face of posts/beams to let the stick framing pass by and become part of the structure.
Again, by all means get an engineer to bless the plan before you cut stick one.
work safe, have fun
deralte