VBox upgrade problems

[Hiro Protagonist]

I have recently moved from Windows 7 to Windows 8. I was using VBox 4.3.6 with an Ubuntu 12.04 guest. I exported the Ubuntu setup as OVA, did the upgrade to Windows 8, installed VBox 4.3.6, and imported the Ubuntu OVA. I now have several problems:
1) NAT no longer has a working DNS. I can ping IP addresses but not domain names. Bridged networking works fine, but I don't want to use bridged. Following advice on various forums, I tried using VBoxManage to set --natdnshostresolver1 on and --natdnsproxy1 on, which had absolutely no effect. I also tried configuring the Ubuntu guest to use DHCP but with 8.8.8.8 as the hand-coded DNS; again, no luck.
2) I tried upgrading Ubuntu 12 to Ubuntu 14 (using bridged networking to access the update server) and ended up with a 640x480 display where the mouse pointer integration no longer worked (the mouse pointer moved in and out of the guest window, but left a mouse pointer active at the edge of the guest window when I moved it out). I tried reinstalling the guest additions but this had no effect. NAT still didn't work.
3) I then went back to the original snapshot and tried installing the available updates to Ubuntu 12 without upgrading. The display was OK (I could resize it) but the same thing happened with the mouse pointer. NAT still didn't work.
4) I uninstalled VBox and installed the latest version (5.0.4) and went through the same sequence (1 to 3) but everything stayed as described above.
5) I installed a brand-new Ubuntu 15.04. Once again I can ping 8.8.8.8 but not www.google.com using NAT, and I'm forced to use bridged networking. Everything else seems fine, but now I need to waste another couple of days configuring and reinstalling all the stuff I had with my old Ubuntu 12 setup

So:
(1) does anyone know how to get NAT working?
(2) is there some magic spell I need to incant to get all the mouse integration etc. working when I upgrade the guest OS?