On the break, all red balls are missed and the cue ball rolls back to the baulk area. Is it a free ball, since all reds are in the original start position and you cannot see both sides of any red?
Depends on the position of the cue ball
If the cue ball is obstructed by a colour from hitting both sides of a red ball - ie it gets tucked up behind the yellow - then yes, it is a free ball as you would expect.
However, if the cue ball ends up, say, over a baulk corner pocket, then no, this would not be a free ball. This is because the player is not deemed to be snookered, and therefore it cannot be given as a free ball.
A Free Ball is awarded to a player when they are snookered after a foul as per Section 3, Rule 12 from the WPBSA Official Rules of the Games of Snooker and English Billiards, which states:
12. Snookered After a Foul
After a foul, if the cue-ball is snookered (see Section 2, Rule 17), the referee shall state FREE BALL.
And being snookered is defined in Section 2, Rule 17 as being:
The cue-ball is said to be snookered when a direct stroke in a straight line to every ball on is wholly or partially obstructed by a ball or balls not on. If one or more balls on can be struck at both extreme edges free of obstruction by any ball not on, the cue-ball is not snookered.
The emphasised text is the key bit here.
If the cue ball is over one of the baulk corner pockets, then the only thing stopping a player from hitting both sides of the corner ball of the pack, is another red (assuming none of the balls were hit). However, this other red ball is also an "on" ball, so it cannot be considered a snookering ball as per Rule 17 above. Therefore no snooker, and hence, no free ball.
No, it is not a free ball.
If one extreme edge of a ball is only obscured by another ball that is also on, no free ball is awarded. All reds are on in the given scenario, so one red can obscure another, it doesn't cause a free ball. Barry Stark, a senior Snooker coach, explains in a video.
Here is a screenshot from the relevant part of the video.