Oh no! Is that over time or a drop? And do you have a pic of the cracking? I got a SI Viper CQB stock but never got round to replacing it because the MTR stock just looks better. But yeah I guess now you see the RS buffer tube on the UBR works! (with some DIY solutions)
Regarding rethreading, you can fit real steel rails & buffer tubes onto the MWS, but you will need to rethread them. As you see from the pic I rethreaded the front of my SBR to add the fortis switch, and I rethreaded the lower receiver on the DMR to fit the UBR stock. I looked at the price but the shop which was selling the g&p UBR repro was only a tad cheaper than the real thing, so I just went for the real thing instead. Plus ubr2>ubr1 XD
The reason I used my MTR lower in my CQB was because it had ambi controls, which IMO are more important in a CQB environment where angles are tight and shoulders are switched often.
When you install the UBR2, you essentially replace everything from the end plate and backwards. You will probably get a better understanding by watching an installation video on youtube.