Arthropod phylogeny

So there is the tadpole shrimp, the horseshoe crab, and the trilobite.
There is Chelicerata and Mandibulata.
There is Decapoda and Hexapoda.
So basic questions.
Are exoskeletons monophyletic?
Is the trilobite more closely related to the tadpole shrimp,
the horseshoe crab, or are the tadpole shrimp and horseshoe
crab more closely related to each other than to the trilobite?
Are sea spiders closely related to land spiders?  Do they have
too many joints in their legs to be closely related?  Are they
a crown group of the arthropods and they have eight legs by
coincidence?  Or are they actually pretty closely related to
land spiders?