OK, I have studied your pictures at length and I cannot come up with any logical reason for what you are describing. In fact, it should be mechanically impossible. When in forward gear the roller should be in the bottom end of the shifter lock and it should be rocked away from the armature plate at the top.
There are only two things I can imagine:
1. You aren't getting it all the way into forward gear. Are you spinning the propeller as you shift so the gears can mesh? You should be.
2. Hate to even suggest this, since you are "mechanically inclined", but any chance you are mistakenly getting forward and reverse mixed up? Nah, I wouldn't think so.
I suspect #1 is the answer. Let us know what you find.
BTW, you say it is fast. That makes me think everything is normal. It wouldn't go fast if it was limited to the lockout position.