Sly, Looks like you are getting a lot of the bases covered, Let me just add a few key points as some might be overlooked. If this pump (and I believe you have stated it is) a Gast 741"R"rocking piston pump and is a 100 psi model with a cfm of only 1.5 it changes things somewhat.A check valve should be built in the unit (pump)already as a back flow does not happen with pistons. The Mix Air type diffusers do not need an external check valve but should have one with low pressure (10 psi) rotary vanes. The Vertex CoActive Airstations have them built in the bladder surface and when the surface slots close the water pressure presses the rubber bladder down on a seat when the power is off so either unit will work. As far as back pressure between a MixAir type and a Bladder type(Vertex) diffuser it will have no effect on this model of pump.( not enough to calculate)You still have to overcome the water pressure regardless.I would be more concerned about the ability to exhaust off (regulate) from any type diffuser to slow down the circulation and perhaps most important monitor your water temps.I agree that putting any type of diffuser in a shallower area will be better but in your size pond if your spring is not adding 40-75 gpm of 55-60F water any type of diffuser placed at any depth running 24/7 will probably warm the water column too much for 350-400 trout.As far as a pressure guage it will tell you when your intake filter needs cleaned and when and if your diffuser is plugging ( that wont happen with the two brands mentioned)or if the line has been pinched so "yes" on the guage.Weighted hose "YEP" Its the ole saying "do it right do it once" You may be able to size down to 3/8 weighted tubing depending on the diffuser you use and the total length needed to save some $$ You may also want to consider adding a summer diffuser that has a riser that puts the diffuser approx 3-4 ft under the water surface to surface circulate.( both produts mentioned can be built that way) It would have the effect of a seperate surface aerator without the cost and would only require a manifold to switch away from the main bottom diffuser, this unit would be run when bottom temps were becoming too high.Hope this helps. Ted