300.5 also specs 4" of concrete allows 0" cover (other than the 4" concrete) if it's wide enough (6" sideways from the conduit each way.) I believe that "equivalent" would allow things like concrete pavers or bricks as well as poured-in-place concrete for in-trench conduit cover.

Not sure how you approached "pressure water" boring - I'd use a section of the conduit size you want to run, or larger, with a section of smaller pipe inside it to run the water in - waste-water and dirt comes out the space between the two pipes, and the larger outer pipe directs the "drilling" - but roots and stones will stop that method, and it's not good for corners.

For tools you only need once, remember rental. But sometimes rental costs as much as having someone come do it.

I can't imagine how you'd envision pouring concrete separate from the trench to be in any way easier or more effective than pouring it in the trench. I can only see that as a complication that makes everything more difficult. Backfill and compact level with the top of the conduit, shape the trench to be wide enough for how deep the conduit is buried, pour the concrete - nothing easier than that if you have to use concrete cover.

You don't have to bond the conduit if it's only there to "provide support or protection of cable assemblies from physical damage", according to 250.86 ex. 2.

National Electrical Code 2014

Chapter 2 Wiring and Protection

Article 250 Grounding and Bonding

250.86 Other Conductor Enclosures and Raceways. Except as permitted by 250.112(I), metal enclosures and raceways for other than service conductors shall be connected to the equipment grounding conductor.

Exception No. 2: Short sections of metal enclosures or raceways used to provide support or protection of cable assemblies from physical damage shall not be required to be connected to the equipment grounding conductor.