Tell me again why I shouldn't wear core shorts all the time

The amount you are swimming is definitely not the point of diminishing returns.  If you spend more time in the pool, and more importantly time on your technique, doing proper workouts, and I would also suggest getting some coaching, you will get much faster.

I wouldn't wear the shorts, if you want the flotation use a pull bouy.

I think the only reason there would possibly be a problem with wearing the core shorts all the time is if you had a technical flaw that the core shorts were hiding, and thus you were potentially limiting your improvement.

For example, if you know you're a leg-sinker that overutilizes a kick for buoyancy rather than propulsion, you may be leaving time on the table by using the core shorts to mask this rather than fixing it once and for all and gaining the extra propulsion and wasting less energy on maintaining body position.

But there are some top coaches like Brett Sutton who (controversially) agree with you - he lets his swimmers use the PB through the whole set, as long as they're in the water working hard. He would argue that unless you have some big technical flaw to fix that the PB/shorts is hiding, go crazy and crank up that swim volume since triathletes don't swim enough, even at the highest levels.

I tend to agree (even though I swim faster without the PB in a pool and I benefit less from wetsuits than others) - swimming more, harder is usually better as a triathlete so if you're enjoying it a lot more with the PB, go for it. But at the same time, keep an eye on trying to address any technical issue that is probably causing this dependency and try and fix it asap.

Because the wetsuit masks deficiencies in the stroke/body position.  And you will be faster in (or out of) the wetsuit if you figure out how to address those deficiencies. 

That's not to say you can't practice in a wetsuit all the time. Just why you shouldn't.

I don't know.  If putting your body in a better position takes the pressure off your shoulder because your recovery is better, then probably wear it and swim more --so you get more pull work done.  But if body position is really causing that shoulder pressure, then maybe fixing your position should be your primary focus so you are not limited with or without the added buoyancy.