My plan is to set up several power districts, 4 districts, use 4 Pwr-56s and 4 CB-1. All sections would be isolated with plastic rail joiners or use the old DC method with common ground and isolate the positive sides into blocks.

This will also allow me to use DC locos (separately of course). Using a DPDT switch to use dual power. Sim to a dcc/dc layout.

Will using multiple PWR56's be a problem ?

Thanks Bill, I couldn't remember the exact name, but you recalled the situation I was trying to rectify perfectly!

Some thoughts from an electronic standpoint.

The desired result of adding an external power supply with greater voltage and current capability, in both examples given where the PWR-56 feeds, or is fed by, said external supply, in reality will not gain you much, if anything.

In the second example (AC->Brick->PWR-56->Ext), the PWR-56 is feeding what appears to be a DC to DC supply.  In this configuration a higher track voltage will be achieved, but the current capacity is still governed by whichever supply has the lower over-current protection point.  If it's the PWR-56, the track output will still be limited to 4 amps.  If the external supply's over-current point is lower, say 3 amps, then the track output will be limited to 3 amps.

In the first (original poster's) configuration (AC->Ext->PWR-56), there are some potential risks.  If the over-current protection of the PWR-56 is located within the unit shown in the picture, again, 4 amps is all that will be available to the track.  Also, as the OP noted, the PWR-56 output voltage is 14 volts and likely can not be adjusted easily, if at all.  In short, this configuration achieves neither desired goal of increased voltage or increased current capacity.  In the worst case scenario, if the PWR-56's over-current protection circuitry is in the brick power supply that comes with it, then feeding the PWR-56 with a supply capable of 7 amps (the LRS-100-15) could conceivably, and more than likely will, damage the PWR-56's output circuitry should a short circuit occur.  Even operating within the capabilities of the external supply, continuously drawing 5 or 6 amps through an unprotected PWR-56 may also damage it.  That is a risk I would not be willing to take.

Conclusion: Adjust the LRS-100-15 output voltage to 16 volts (it will go that high, as I have done this on my LRS-100-15 I use to power my Digitrax 8-amp Super Chief Command Station) and connect it directly to the track.  You'll get 16 volts with up to 7 amps available to run those O scale trains.  If you still need the repeater function of the PWR-56, just place it where needed, plug it in, and don't worry about connecting the output to anything.

Don't want to leave you hanging....
There wasn't much else to do other than plug the light harnesses together, tuck everything into place and secure things with a bit a Kapton tape here and there.
All's good!

Actually, can't take credit for that .... Digitrax (dirty word) has 1" version of the 8 to 9 pin harnesses ... Called "medium short harness"

https://www.digitrax.com/products/mobile-decoders/dhwhps/