Do cut edges on fiber cement siding really need to be sealed?

Yes, a rough edge soaks up water faster, and scoring and snapping actually leaves you with a better edge than a saw blade does. The snapped edge is denser and less fuzzy than a sawn one, so you're in decent shape there. The chewed up spots are the ones to worry about, since those little frayed fibers wick moisture like a paint roller. Your neighbor prolly got away with it because of decent overhangs and a sheltered wall, but back walls of garages take weather beating and the bottom courses especially catch splash back. Skip the sealing and you might not see problems for years, or you might get swelling and delamination at the edges in five. It's a gamble with a long payout timeline. Here's how to not lose a whole weekend to it: don't seal the edges on the ladder. Cut all your boards first, lay them flat on a couple sawhorses, and roll or brush primer on every cut edge and end while they're sitting right there in front of you. Takes maybe an hour for a garage wall. The half gallon you've allready got is plenty since edges only need a thin coat. Then hanging day is just hanging. Do that and you're following the install sheet without the extra weekend. On the gaps, an eighth of an inch is what the sheet calls for and a couple tight spots won't hurt anything. Fiber cement barely moves with the seasons compared to wood, so the boards aren't going to expand into each other and buckle. What you want to avoid is forcing boards hard against each other with no gap at all, and it sounds like you're nowhere near that. If a tight joint bugs you visually, a thin bead of paintable caulk there is fine, but I wouldn't go around prying boards apart to chase a perfect gap. Check your nail placement is per the instructions and you're solid.

One thing I learned doing my shed the same way is that the edges aren't done once the boards go up. Handling them on a ladder chips corners and the fastener heads leave little craters, so I kept the primer and a cheap 1 inch brush in a cup on the ladder rail and touched up every nick as I went. Took seconds per board and saved me a second trip later. Also, once the primer has dried on those cut ends, hit them with your top coat color before or right after hanging, because bare primer alone isn't the finish the manufacturer is picturing when it says seal the edges.

Agree with doing all the priming on sawhorses first, that's exactly how I did my shed siding and it turned a chore into basically nothing. One thing that helped me was priming the boards in the shade of the garage instead of out in the sun. The primer was drying before I could finish a batch of edges that first day, and dry edges on a stack of boards stick together and peel when you grab them. Moved everything into the shade and it went smooth. Also flip your stack between coats if the ends drink it in fast, those end grains take two passes sometimes.

Update: priming everything flat before hanging was the move. Way faster than my ladder plan, edges are done in one sitting. What I'm still chewing on is those tight spots where boards nearly touch. Anyone had that bite them after a hot summer, or do they just sit there fine? Debating wheather to run a blade thru and open them back up.