Other than the Doomsday Device (and even that's situational), I'd so use all of those ideas.
The No-Turn reminds me of the old nail grenades from QWTF.
Are you mad?
the nail gun wouldn't work because it has to be placed in a position where a static building is most vulnerable, in order to be most effective; the centre of a room. People don't flank a sentry gun from all directions, they usually assault from a single direction using each other as cover, hammering it out of position with sheer fire power or ubers. It supposedly has low health and damages the engi too, so he can't even heal it in combat. The thing would drop like a fly. Unless it could be thrown and had a 1 second setup time this thing would be useless compared to a sentry. You'd be better off just going demoman.
As for the crit device. I can only see it being viable in A/D, and even then dispensers are too valuble to a players life expectancy to realisticly trade it out, not to mention the fact it would drasticly decrease the effectiveness of sentry nests. When you have medics on your team dishing out kritz and invulns this device is nothing but redundant. Not to mention the fact that as a static building it reduces the mobility of your team. At least people leave the dispensers side to complete objectives after healing up. you'd be more valueble to your team overhealing as a medic.
speaking as an experienced engineer whose class has his highest score, and a soldier/demoman/spy as highest played times, who ends up clearing out sentry nests frequently, all of these concepts fail the engineer as useful dynamic tools; each which can be out performed by an individual class or fail to support a team in any effective manner equal to that of his current gear.