LNB one bad feed

Not likely.  Did you try swapping the two coax leads on the back of the HR23 where the problem is?  If the problem stays with the same tuner, then the HR23 is bad.

No, it's incredibly unlikely that two LNBs would have this sort of failure. Do the cables from the dish go through a grounding block, or surge protector somewhere?

Seems like it can only be the LNB.  Where did you get the "new" LNB?  How old was the original LNB?

I can only suggest you try another LNB.  Or call for a service visit.  It's only $50 without the Equipment Protection Plan. 

I agree.  The $50 service call would be cheaper than the LNB.  I'm pretty well baffled by changing out a non-SWM system to a SWM system, then back again.  What was the point of that?

There's a whole lot more to going from a SWM to a non-SWM system (and vice-versa) than changing out the LNB.  If you don't understand that, then you REALLY need a service call.

This is the confusing statement you made ....

Replacement LNB from changeout of service to swm system but was working fine before changeout.

I was also wondering if you changed the LNB type.  Any chance you can test one or both of those "bad" LNBs in somebody else's dish?  Give the troubleshooting you've done, it seems it can only be a defective LNB.

When the solution to any problem seems impossible to locate and the evidence doesn't make sense, it's time to get an expert.  If it were me I'd spend the fifty bucks and let the tech locate the problem.

Is it possible that the "second port from the right" of both LNBs was connected to the same tuner when they failed?  I think I would check voltages at the satellite in ports before the tech installs a third LNB.  Should be a max of about 18 vdc.