POE to remote location as the sole DC power supply

The situation is such that I'm running POE+ to a remote location with very limited options for leveraging another power supply. So I have an Ethernet cable with POE connected to a POE splitter that separates the power from the data (it is 802.3af/at compliant).

The splitter I'm using is this one: https://www.amazon.com/gp/product/B08HS4NT13/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&th=1. The reason I selected this model is because they claim it is electrically isolated to 1500V.

The DC is split out onto a barrel connector. I then have that splitter connected to a simple Y-cable so that there are 2 DC barrel connectors on the ends.

One end I connect to the barrel connector on the Arduino Mega 2560 with POE, and the Ethernet cable connects to the Arduino, which now starts up properly.

Then I have a wire from 5V pin on Arduino to DC+ on the opto-isolated low/high trigger relay module. Then I have a wire from GND on the Arduino to DC- on that same relay module. This is a link to the opto-isolated low/high trigger relay module: https://www.amazon.com/gp/product/B00LW15A4W/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

The second barrel connector is terminated to a breakout so that I can run wires from that breakout's 12VDC+ to Common on the relay contact. Then I have a 12VDC horn that's connected to NC. The horn draws a max of 600mA.

I have very few options for bringing another power supply. THere's simply nowhere to plug into. And this location is literally in the middle of nowhere. Is there anything I can do to keep the output circuit from damaging the control circuit?

I read that when standard compliant POE device does DC-DC conversion, that's considered isolation. And I also read that POE compliant switch ports are required to be electrically isolated (I'm using a Cisco 9200 24 port POE+ switch). But I'm not sure if that takes into account extenders and splitters.

Then I have one of the Arduino pins (12) programmed to go high when some condition is met. This occurs, and 5VDC activates the coil and switches the relay NC/NO contact.

However, the more I read (and obsess), I'm pretty confident that this isn't a good practice, and I'm worried that some issue on the output side will make its way back to the micro-controller and kill it. I'm looking for input and guidance.

It seems to me that it would be safer to run a second POE cable to the remote location. Of course, I'd still then have to rely on the manufacturers to be following the standard for port isolation. Running that cable would come at an expense, but if that's the recommended action, I can push it up the ladder.

Also, as a side note, the switch is POE+ (30W max output per port).

I saw this and thought "that might be an interesting question to answer", but unfortunately I cannot understand what you have done from your description. Please post a schematic. Descriptions of this connected to that and the other are useless. The language of electronics is a schematic or circuit diagram. Hand drawn and photographed is perfectly acceptable.

Thank you.

A schematic may be useful to other helpers.

I'm not familiar with how exactly POE does its thing. And, even if the specifics say it should be isolated, you don't know what costs may be cut in your module.
If you want to be 100% sure, you can try to roll out your own solution.
Since it is a remote location, i would suggest doing it similar to the mains grid - generate higher voltage (but not too high to be unsafe) AC at the source, send that through a separate cable (you said you can consider running another cable), then rectify and stabilize it at the destination.

What distance are we talking about?

Since I'm using the SIlvertel AG9600 (5V POE module), I'm trying to find out if I can replace that with a 12V version and then connect directly to pins 7 (VDC+) and 8 (VDC-) on that module and use that to power the horn that's connected the the relay contacts.

Looking at this, the 12V version provides about half the current of the 5V version - 1A vs. 1.8A
If both the Arduino (800mA) and the horn (600mA) hit max consumption at the same time, it may cause troubles.
Also, if you power the Arduino with 12V, the onboard voltage regulator will get pretty hot.

Have you considered using a boost converter for the horn? This will also provide another layer of isolation.

Advisable not to use "Vin" or the "barrel jack" of the older Arduinos for any actual project since the regulator is grossly inadequate.