I understand your current system is as follows
PC —> Serial —> Current Loop --> Machine
And you would like to have it as below
PC ------> (Wifi) Photon —> Serial -> Current Loop —> Machine
Ultimately you would like to have many Machines connected to a single PC through multiple photons
PC -----> (Over Wi-Fi) Photon-1 ----> Serial -> Current Loop --> Machine-1
------> (Over Wi-Fi) Photon-2 —> Serial --> Current Loop —> Machine-2
------> (Over Wi-Fi) Photon-n —> Serial --> Current Loop —> Machine-n
To your question #2, each Photon has a unique device ID and access token. You can connect to the Photon using TCP/UDP. Using TCP/UDP will be local as @kennethlimcp mentioned. You can also use Particles REST APIs to send information to Photon, but this goes over the cloud. Please check http://docs.particle.io/core/api/
To your question #3, please check if the link https://www.cooking-hacks.com/documentation/tutorials/4-20ma-sensor-board-shield-tutorial-for-arduino-raspberry-pi-intel-galileo is useful. This is done for Arduino, but must be applicable to photon as well. @kennethlimcp will have better suggestions.
To your question #1, reliability can be trusted with TCP. With UDP, you need to build it into your PC side and Photon side software.