I normally use .PSD's not TIFF's when I round trip to PS. I have never had any problems with going back to PS again. As long as I use Edit Original file (the bottom option) the image auto updates both the "original" and VC's. Of course you have to use save not save as in PS to maintain the link. I often have several VC's for different print sizes, or a mono conversion (I perfer to do mono conversions in LR, and for some tricky occasions I have actually round tripped to PS to fix the colour version before converting to mono). I do not ever save .xmp files. I'm using LR4.4.
There are three options for opening existing RGB files. Open a copy, which opens a new version of the file, if the file has layers then the new file maintains those layers. If you chose copy with "LR Edits" then a new file is created, and you simply get a new background layer. In both of these situations, if the original file was from a RAW file you would get a file name in the format "Original filename-XXX-YYY.psd (or tiff) where XXX is the first 3 digit number that gets added when you create a file and the YYY is the second application of that numerical sequence. In both these cases the link to the original RGB file is broken, although it will appeare stacked with the original files.
Choosing the last option, Edit Original, as I said above should simply open the original file. When it is updated it should automatically update. As I use an older version of ACR I have to have LR render all of my RAW's when I open them in PS, so I always get to see the new .psd file in LR before PS opens. When I go back to LR it may take a few moments before the preview image updates. On some occasions where I have VC's you have to do something within LR that makes the system update the previews before the PS changes are shown. It may just be that LR is not imidiatley updateing your preview images.
I have recently been playing with some new processing techniques, so have been opening quite a few older images in PS.
Alan