Are any letters commonly missing from the set?
It happens often enough to be a real pattern rather than a rare fluke. Check your set against a full 52-piece alphabet, 26 uppercase and 26 lowercase, immediately after opening rather than assuming completeness, and raise it with the seller promptly if anything is missing.
How many letters are in the box?
52: a full uppercase and lowercase alphabet, 26 of each, in a range of colors. There is only one of each letter, so words with repeated letters, such as book or apple, need a second set or a little creativity. Count them against a full alphabet when the box arrives, before the first play session.
Are the magnets strong enough to stay on the fridge?
Usually, but hold strength varies by piece and by set. The magnets are thin, flat sheets across the back rather than powerful discs, so expect adequate holding power on a steel fridge door rather than an exceptionally strong grip.
How durable are these for a toddler who plays with them daily?
Durability is less consistent than the build suggests, but a set can still have working magnets after two and a half years of regular use. Expect a well-made wooden product that can show some wear with heavy daily toddler use rather than one immune to it.
What does the paint or finish involve? Is it safe for young kids to chew or mouth?
Melissa & Doug describes the letters as child-safe wood with smooth edges for small hands, aimed at ages 3 to 7, but it does not detail the paint or finish or name a paint safety certification. The colored letter face may be a printed layer glued to the wood, another reason not to let a child chew them, so check the packaging for any stated certifications if this matters for your child's age or habits.
Is the magnet design safer than plastic alphabet magnets with small embedded magnets?
Largely, yes. The magnet on each letter covers the entire back of the piece rather than sitting in a small embedded disc, so there is no small loose magnet to become a separate swallowing hazard. That is a practical advantage rather than a certified safety feature, and the letters are still small enough to be a choking risk, so no child who chews should be left alone with them.