Can you explain more of the issue that you are having?

In most cases, you do not need iTunes to synch music, even with the SE https://www.imobie.com/support/how-to-add-music-to-iphone-se.htm.

The SE is fairly low-level in its needs, so curious to see what the challenge is.

I think you are SOL. Even with the low end requirements of the SEyou need to run at least Mavericks, which does not support your 2006 (requirements below).

Outside of buying a new computer, you could look at some cloud syncing software or backup software to synchronize your 2008 computer to your 2006 computer.

OS X Mavericks system requirements
Learn about the system requirements for OS X Mavericks v10.9.

This article has been archived and is no longer updated by Apple.
To install Mavericks, you need one of these Macs:
iMac (Mid-2007 or later)
MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
MacBook Pro (13-inch, Mid-2009 or later)
MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
MacBook Air (Late 2008 or later)
Mac mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
Xserve (Early 2009)
Your Mac also needs:
OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
2 GB or more of memory
8 GB or more of available space (Learn how to increase disk space)

Re-reading this, the one thing I don't understand is that the upload worked before he got the cell service turned on. OP, what changed in the day from when you were uploading your pics to you turning on your phone service that prevents you from uploading your pics?

For your image import this is not a problem. When you activated your phone, you probably set the option for a passcode. When you plugged your phone into the computer, you are are getting the message from your picture. It is asking you to put in your passcode on your phone to validate who you are and your connection. You can always get rid of the passcode in your settings. Try that and see if that helps you.

As for using older versions of software, check out this article below. It shows you how to download Lion from the store since you paid for it. I think that this is less of a think where Apple is trying to screw with you and more of a case where your older gear can only be updated so far. Lion was released in 2011 and has been sunset--hence the reason that you rain into the issues that your did.

https://www.macworld.co.uk/how-to/mac-software/download-old-os-x-3629363/

What specific error messages are you getting? Based upon that I can try to help you out. Otherwise, keep in mind you have Genius Bar help from Apple (in person and virtual). They can probably help you out as well. https://getsupport.apple.com/?caller=psp&PRKEYS=PL133

This is the point that continues to confuse me. Why do you keep saying by design? The phone does not care if it has a data plan or not when it is trying to load the pictures to your photos app. If there was a version compatibility issue, it would have surfaced when you were able to synch when you were only on wifi. Activating the phone would not make any changes that should impact compatibility.

Can you post pics of the specific screens you are getting on the Mac and Phone? The one that you posted above was about entering the passcode on the phone. That has nothing to do with the version of SW you are using. You keep alluding to Apple playing tricks on you because of your older hardware. Apple does not allude. They will either tell you of a version incompatibility issue or it will just work. Apple is not subtle in the least.

Alternatively, reach out directly to Apple. Their tech support is great and they can walk you through things.