Let me try and answer these one by one:
The car deck is more or less the same width - in fact if anything I think it might be a couple of inches wider. I know in her latter days on the Solent it was more common to put three coaches abreast on the Cuthred than it was the Caedmon. Any difference though is negligible/
The windows overlooking the 2 side lounges were unique to Cuthred - they made quite a good viewing point when the vessel was loading up. Even when the port side lounge was replaced by a second set of ladies loos the windows were retained, though obviously they were on the corridor side, not directly into the loos!!
The stairways were both at the stern end of the ship - Portsmouth end. They came up into the bar area at each corner, almost adjacent to the doors out on to the open deck
Yes, there are two lower lounges - as usual hardly ever used. They had softer padded seats in them, the same as in the side deck lounges, rather than the hard plastic found on the later vessels. One was furnished in orange seats, the other in green I believe. When she ran in Portugal, these were closed off to the public and used for storage. They were also much less well-lit than on the C-class - very dark and dingy, which was handy if you wanted a quick sleep during the crossing
Hope that helps - I'm still trying to find the photos I took when on her in Portugal; as soon as I get them, I will post them