Routing traffic back and forth through a Mac mini server with 2 network interfaces
It should be possible to create a bridge with en0 and en2 and enable net.inet.ip.forwarding to get rid of all routing problems. The bridge acts more or less as another switch between en0 and en2. 




Disable Internet Sharing
remove the gateway in the en2 settings of the Mac mini and change the IP-address to an available one in 192.168.88.0/24 (e.g. 192.168.88.11)
remove all additional static routes
Check for bridge interfaces with ifconfig
On the server create a file bridge:

sudo mkdir -p /usr/local/bin/ #only if the folder is missing
sudo nano /usr/local/bin/bridge


with the content

#!/bin/bash

sysctl -w net.inet.ip.forwarding=1
ifconfig bridge create
ifconfig bridge0 addm en0 addm en2 up #use the first available bridge number here and the proper interface device names

enter sudo chmod +x nano /usr/local/bin/bridge
Create a launch daemon usr.bridge.plist:

sudo nano /Library/LaunchDaemons/usr.bridge.plist


with the content

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;usr.bridge&lt;/string&gt;
    &lt;key&gt;ProgramArguments&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;/bin/sh&lt;/string&gt;
        &lt;string&gt;-c&lt;/string&gt;
        &lt;string&gt;/usr/local/bin/bridge&lt;/string&gt;
    &lt;/array&gt;
    &lt;key&gt;RunAtLoad&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;StandardErrorPath&lt;/key&gt;
    &lt;string&gt;/tmp/usr.bridge.err&lt;/string&gt;
    &lt;key&gt;StandardOutPath&lt;/key&gt;
    &lt;string&gt;/tmp/usr.bridge.out&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;

Load the plist 

sudo launchctl load /Library/LaunchDaemons/usr.bridge.plist

Change the IP-addresses of node_1 and node_10 to available ones in 192.168.88.0/24 (e.g. 192.168.88.101 and 192.168.88.110). Change their default gateways to 192.168.88.1 and the DNS server to 192.168.88.10.