No it decreases dps because your team's damage drops off significantly, (as they aren't attacking). It's better to switch to another character, and let the ai attack super fast, but not use any of your important abilities in the process. When you use flask of lightning, it's primarily a defensive ability, allowing you to easily get into flanking position, dodge projectiles that can be dodged, and avoid other dangers. When the ai uses flask of lightning, it increases your attack speed making it more offensive, though it does have defensive benefits depending on your defensive masterworks.
Flank Attack is good because it lets you enter stealth every 8 seconds, dropping aggro when your frost flask is on cooldown, and activating the extremely useful ambush. 50% armor penetration for 6 seconds is too good to pass up and the stealth attack bonus is significant. I love poisoned weapons, but unless I'm doing my artificer or assassin build, I usually forgo it.
One you have Sneak Attack working to a significant degree, Mercy Killing becomes partially redundant, though Knockout Powder never really loses it's usefulness.
It's important to note that mob killing and boss killing has extremely different builds for a tempest, due to Pincushion being not that great on normal mobs, but really good on bosses and getting it means you have to switch around your build a little bit. ( I get rid of the left hand side of the Tempest tree, as you can't freeze bosses)
As far as throwing knives go, I have no idea how good it is, because I don't use it. I never really go that far into the Sabotage tree. ( though I do get the "looks like it hurt," and eventually "cheap shots" passives). All I can say is that if you find it good, use it, if not, don't.
Deathblow will never replace Twin Fangs because the latter has more reliable damage based on flanking meaning it will do more damage in most scenarios, plus both are precision detonators meaning they will both get use in a tempest playstyle. Shadow Strike is also a precision detonator while also being a great nuke, which means that must be taken into account when deciding how good it is. Combined with how good it is with Flask of Fire, I think it's the most important non flask ability to a tempest.