I wouldn't because then you're relying on outside network infrastructure for your security.
I'd recommend creating an internal proxy server that injects credentials.
In reality, this will probably work just fine, but if it were something highly sensitive, I'd go with my previous recommendation, it's pretty easy to setup.
Yes and No.
Yes - because you will be safe from non-targeted external attacks (if your website does have proper settings for not leaking data on denied requests, etc, but that was not the question). It's almost impossible to spoof IP from outside the perimeter (not being man in the middle), however still possible, but that would require quite a lot of ...
The question asked is "Is it acceptable in this case to rely on IP as an authentication [mechanism]?"
The short answer is: It depends.
Let's start with a definition of authentication. Merriam-Webster defines authentication as
": an act, process, or method of showing something (such as an
identity, a piece of art, or a financial transaction) to be ...
You should use IP address for authorization, but not for authentication. Authentication is who you are, authorization is what you are allowed to do.
If you use IP address for access control, you don't have authentication. You don't know who used what, and any audit trail is useless. But it works for simpler cases, like a read-only documentation site for ...
Is it acceptable in this case to rely on IP as an authentication mechanism ?
Yes, because performing an attack that will spoof a connection requires the attacker to be placed in a very very specific location along the path of communication. So you are safe from that.
The above holds, assuming you don't have any confidential data and you just want to allow ...