1.Stats: Should Cloyster get a stat boost, Evolution, Mega Evolution or nothing?
Historically, Cloyster had the same BST as Gyarados, 480. This would be 565 BST, if it retained 85 Special Defense, or 535 BST, to match Gyarados' 540, or 580, to match the general change from Gen I fully evolved BST to Gen II fully evolved BST, where the other Special stat would be at 100(which for Cloyster would be 100 Special Defense).
However, Cloyster already has 220 BST gap, increasing this to 230 BST will not help much.
Because the gap and the current BST are both too large, there is less of a point of an evolution, and more of a point for a middle-evolution, around 415 BST. But that won't help things either.
This leaves a Mega, or an alternate form similar to Slowbro's Shellder would be the best solution for hyper-specialization of Cloyster.
2.Should Cloyster get more abilities?
Skill Link and Shell Armor are both decent abilities, but there are far better abilities Cloyster can get in addition to full three set it already has.Ice Body, Snow Cloak, Soundproof, Water Absorb, Water Veil, Weak Armor would be fitting abilities. Technician, Drizzle and Snow Warning can also work. Because Cloyster has 70 Speed, Slush Rush and Swift Swim can also work.
3. Should Cloyster get more moves?
Yes. Most obvious missing move for Cloyster would be Swords Dance, Recover, Rock Slide, Rock Tomb, Bulldoze, Assurance, Beat Up, Bullet Seed, Fell Stinger, Water Spout, Mist, Water Shuriken and Power Gem, among others
4.What additional typing Cloyster can have?
Poison, Dark, Steel and Normal are the most obvious ones. Rock and Ground would be lesser.