Case does that on the quad trac.  Lots of Cat, JCB, Kobelco and probably lots of other planetary finals use preset bearing sets like that for wheel bearings.   One of the latest fleet magazines had an article indicating that it's the "new thing" for semi trailer axles.  

Personally I would rather have a spindle with lock nuts that the preload can be easily adjusted on.  

I know one person who has used shims to adjust preload on such a setup.   I'm not all that fond of shims in single digit mil thicknesses, either.  .

Regarding using gun grease.....looks like a sure way to ruin the duo-cone seals.  They depend on oil getting far enough out to lube the seal.  Necessary viscosity is dependent on rotational speed.  That's why the idler has different oil requirement then the midwheels.  

Putting the seals together without a tool is not impossible.  Probably not advisable either, but with care it most certainly can be done.  BTDT. 

There is a very informative, detailed Cat paper covering the theory of operation of the duo-cone seal online which can be found by googling cat duo cone seals.  It also has dimensions for all Cat numbers.  There are aftermarket sources which are a fraction of the cost from Caterpillar.  Caterpillar itself is a fraction of the cost from CaseIH.