I was doing some testing with Sniper's Mark split projectiles + wall chains from Deadeye's new Ricochet node in conjunction with spectral shield throw.

Here's what I found:

• Projectiles CAN split off the primary target, then chain off the wall back to the same target; however, they deal no damage and don't seem to proc on-hit effects when they chain back. This seems to also prevent secondary skill effects like lightning arrow's lightning strikes or kinetic blast explosions (but needs more testing).
• Chaining off hydrosphere will proc secondary skill effects, including Spectral Shield Throw (SST)'s secondary shard projectiles. These shards can also hit the primary target in addition to the main projectile, resulting in 75% more damage.
• You can chain off hydrosphere placed at a distance to guarantee far shot bonus damage (60% more damage). Even if the enemy is close to you, far shot damage is based off of how far the projectile has actually traveled, not how close the enemy is. The Fledgling helmet is a good source of far shot if you don't want to pick it up in the Deadeye ascendancy.
• Forgot to mention in the video, your chains will never target the hydrosphere. It MUST be the first target hit, you can't chain back to it.

Other interactions:

• The Wandslinger notable allows you to scale SST (or shield charge) damage with spell damage if you're wielding a wand in your main hand.
• The new Sinistral Gloves base type gives up to 50% increased damage with offhand but 25% reduced damage with mainhand. The offhand damage works for SST (or shield charge) and you can ignore the downside since we don't attack with our mainhand.

Basic PoB for Impale SST Deadeye: https://pastebin.com/QffKBmm9