TOO LONG: DID NOT READ. Your computer is too old, and even if you can get it online, the drivers you want won't work for your equipment. You'll need a Mac Pro 2010 at the earliest if you want native support, or a Mac Pro 2009 if you're willing to hack/ get messy with the command line.
Here's the detailed answers:
1. According to the excellent MacTracker, the first MacPro is able to go up to Mac OS X 10.7.5 ("Lion") - one version above Snow Leopard (Mac OS X 10.6). You should be able to find a copy of Lion for sale, it was sold on USB sticks by Apple at one time - or they MIGHT still sell it at stores.
2. You should be able to install Windows 10 on your MacPro (see:
https://forums.macrumors.com/thread...1-2-1-without-usb-or-dvd-or-bootcamp.2151782/ for how to do it without bootcamp). However, remember that Macintosh programs will not work under Windows, and your MacPro is a Windows machine.
3. I'm not sure where the errors are coming from:
>"establish a secure connection"
Is that message when you open a website using Safari? I don't think Safari for Snow Leopard uses the latest security protocols, and a lot of websites won't open with Safari. You should be able to download a (more recent) version of FireFox that will let you connect to websites (but don't expect it to last too much longer).
If this message is when you're connecting to the internet, could it be because the machine doesn't know the protocol of the wifi network? For example, do you use WPA 3 when your system can only use WEP?
>"your clock is wrong"
Is the date/month/year set correctly on your computer?
Out of curiosity, I decided to go to PreSonus's website.
1. It requires an HTTPS connection. I don't know if Snow Leopard can connect to websites with HTTPS connections.
2. The website is extremely modern, and I doubt Snow Leopard's Safari can render it correctly.
3. I don't think the software would work on your machine.
- Studio One 6 and Studio One+ requires MacOS 10.14 (
https://www.presonus.com/en-US/studio-one-tech-specs.html) or an i3 processor for Windows.
- Capture requires MacOS 10.13 64 bit only or an i3 Processor for Windows (
https://legacy.presonus.com/products/Capture/tech-specs)
- Notion requires Mac OS 10.13 64 bit or an i3 Processor for Windows (
https://legacy.presonus.com/products/Notion/tech-specs)
I'm not able to find old version of their software that might be compatible with that machine.
For drivers, I decided to look at the Studio Live Series 3 mixer. It requires MacOS 10.14 (
https://www.presonus.com/en-US/mixer-system-requirements.html) or 64 bit version of Windows 10. Now why you're using a $2,000 USD mixer on an old computer I don't know...
Likewise, the StudioLive Series of mixers also requires MacOS 10.14 or Windows 10, 64 bit. (
https://www.presonus.com/en-US/usb-audio-interface-system-requirements.html)
Honestly, you need a minimum of a Mid 2010 MacPro to run this equipment (since a 2010 MacPro can run MacOS 10.14). PreSonus's program, "Universal Control", requires at least 10.13 (source:
https://legacy.presonus.com/products/Universal-Control/downloads)