yes, spike traps can crit, and they already have a high weapon damage %, so crits are huge. That is why I crit is still important, it will add a lot of burst damage with the traps
Well,to maximize damage with sentry, you need to make the mobs stand still, because you will want to set up 3 sentry, in a way that all of then can deal damage. In order to do that, they need to be somewhat close to each other, and they need to be set up as quick as possible. With that in mind, lets do some math: with the 8 seconds cooldown, you take 16 seconds to lay down all 3 sentry (lay down the first one, wait 8 seconds, lay down seconds one, wait more 8 seconds, lay down third one). If you are kiting, in 16 seconds, the third sentry will be pretty far away from the first one, and as the monsters follow you, they move out of range of the first sentry. So, you will never be able to set up all traps hitting the demons at the same time, which defeats the purpose of the build.
That is why spike trap: echoing blast works nice with this build. Lay your sentry, pop gloom, lay down two or three spike traps at your feet, if you want, you can move a little, set seconds trap, more spike traps, use your generator a little to build up your hatred, keep laying spike traps and sentry as they expire. Rinse and Repeat. Remember to keep up gloom all the time if you are taking heavy damage, and to activate it before up put the traps down, or else you won't get life back. Facetanking also helps echoing blast passive, because this way the monster are standing still, and each trap can hit three times.
Of course, I am assuming you are using custom engineering passive, which allows you to put down one more sentry and 3 more spike traps.
If you can't facetank, I would pick stun grenades as a generator, as you can somewhat stun lock with then, spike traps: scatter, and maybe rain of vengeance with stun rune (dont remember the name) or the knockback rune(stampede). This way you can set up the sentry, move out of the way, and keep mobs standing still with stun grenades and rain of vengeance.