Don't understand HSPL needing Unlock first

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Here's my story,

I had a stock HTC Surround on TELUS that was upgraded to 7.8, but I wanted to install a custom rom. I downgraded my SPL by using the TELUS Test ROM available off the web (link escapes me at the moment). This brought my SPL down to 1.4.220.2(113577) and WP7 to 7004.

When I went to install RSPL, (so I could then HSPL), the phone wouldn't reboot like it's supposed to. It just stayed in the RBG screen

Now, if I try and install either the Cingular SPL or ROM using RUU, it fails, stating Invalid Vendor ID.

I tried using the goldcard method and Cingular SPL or ROM freeze during 'Loading..'. TELUS Test ROM from above works perfectly (So I know my GoldCard is at least working)

If I try and upgrade my phone by either using Zune or Cab Sender, it successfully upgrades from 7004 to Nodo, but fails trying to get to Mango:
ERROR: E_CANNOT_UPDATE_TO_HIGHEST_VERSION: CreatePaths: For package named: QC_8x50_ULDR_BOOT cannot find a path to highest expected version of: 2250.21.15204.502
ERROR: E_INVALID_SIGNATURE:
Package: \OSRoot\Application Data\Microsoft\DeviceUpdate\Packages\1F43F151-0D31-4291-B316-7018956578FD.1.pks\QC_8X50_ULDR.cab.pkg FROM Version: 0.0.0.0 TO Version: 2250.21.15204.502 GUID = {6939BB9F-56E4-47F3-AF2A-01DDFB414825}

There's a whole bunch more, with the same type of error. I belive it may have something to do with the firmware version.

I tried using the OEM cab files available on the web, but they're for Carrier 502, and I'm on 661, so they fail as well

So here I am, stuck at Nodo and unable to install HSPL to get a custom ROM.

I created a thread in the Surround Forum, but it's pretty dead there, and I had a few replies, but they died quickly (http://forum.xda-developers.com/show....php?t=2141523).

One guy mentioned that I needed to unlock my phone before HSPL would work, but I don't know what he means by that. I thought that HSPL was used to unlock the phone. Is he meaning IMEI unlock? I don't understand. This is my main question: What unlock is he talking about, and how can I get RSPL installed on my phone.

The latest thing is that I've managed to developer unlock my phone, if that helps at all.

Thanks for your guys input. I really appreciate it!