Both of those sound reasonable, but there are way more options.
First, let's talk about offense.
Arthropods have already come up with some amazing weaponry on their own. Meet the mantis shrimp:
Properly known as stomatopods, they are beautiful and they are deadly. The rounded appendages you see are actually the best "smashers" in nature. The mantis shrimp stores a massive amount of energy in a saddle-shaped "spring" made of chitin and then releases it all at once, accelerating the spring forward with incredible velocity. As you point out in the question, water is hard to move through, but the mantis shrimp releases the energy so quickly that the water doesn't have time to move out of the way- so it cavitates. This creates a small bubble of low pressure right behind the smasher that collapses, creating a second shockwave to truly decimate the shell of the organism that it's smashing. If you google it, there are a plethora of videos showing exactly this behavior. If smashing isn't your style, there are also "spearers" who have pointy appendages that let them spear fish and other organisms at nearly the speed of sound. Oh, and neither type can be kept in normal aquarium tanks because they'll smash through the glass.
If smashing through your opponent's shell wasn't brilliant enough, they can use create some incredibly sharp shell pieces that will allow them to pry apart any enemies. Now, the horseshoe crabs might not use these for offense, but as someone who's accidentally stepped on a shell, they're plenty sharp. This type of attack would look like fencing, with each Crabite trying to get their lance inside the joints of the other's shell.
If your Crabites are capable of producing and using plastic, they're clearly fairly advanced and would be capable of synthesizing things like phosphoric acid or a number of strong bases. These chemicals will dissolve straight through an opponent's shell. While it might be hard to deploy these weapons given their tendency to get, well, everywhere, it might be best to coat weapons in these chemicals before their use. Those weapons would have to be plastic, however- not chitin.
Now, these creatures are at war with each other, and are presumably not held back by simple matters such as the Biological Weapons Convention. This allows them to get up to all kinds of nasty stuff, especially given that they likely live near extreme environments that encourage bacteria and archaea to do strange things. Some applications I can imagine are infecting the other side with parasites, releasing crab viruses into the water "upstream" of them, or finding flat-out toxic bacteria to release.
Your Crabites shouldn't just worry about just the offensive side of things. They can take some measures to defend themselves from other organisms, including other Crabites.
The Crabites would do well to forge plastic armor as soon as possible. Plastic is actually a much better defense than chitin- while chitin sticks around on beaches for maybe a year before breaking down, some kinds of plastic will be there forever. Plastic also can be neutrally buoyant, giving them some defense without additional weight.
They might also team up with deep-sea anemones to provide some additional protection.
2) Chemical defense
Of course, a thick shell isn't much use when you have creatures like grey whales who will chomp through entire sections of seafloor. Instead, the Crabites might dine on cyanobacteria or algae to gain poisonous capabilities. These would be all the more effective on large organisms like whales, sea otters, or humans.
If the Crabites need a quick getaway, perhaps they'll take a book from the squid or octopi and leave in a cloud of ink, which could be harvested from any cephalopods they find on the seafloor. If that isn't cool enough, it might be bioluminescent if harvested from the deep sea vampire squid- a biological flash grenade.
I'll just leave this here.
Your Crabites are a terrifying threat. Let's hope they never happen...