[MIKEROPHONICS]

that's a ton of questions....

1) Nugen Halo or Penteo by Perfect Sound are the best choices for upmixers.
Best results come from treating the stems separately or using your stereo session as the source and running quite a few instances of the upmixers. But having said that, perfectly acceptable results can be had (despite the the cries of "oh-no" from the peanut gallery) by upmixing the stereo mix only, as long as you are careful and not silly. These plugs are very clever.

The key to this all is understanding your delivery requirements.

2) WLM? Can;t comment as I use Nugen for metering (they are 'the daddy of loudness plugs'!).

Make seperate stem record tracks for your deliverables (such as Dx, Mx, Fx, DME, M&E etc). Pro Tools will never internally make polywavs, just multitrack surround tracks that play the collection of mono L, R, C, LFE, Ls, Rs etc. Meters can be put on these record tracks and you can monitor in input safely these days. Personally I use stem monitoring by Neyrinck to simulate the DFC.

I never send out interleaved files as PT can't measure them, I send out mono files.

Presumably you will have mixed the stereo version to your delivery loudness specs before going to the other studio? Listen to your mix in stereo there, get familiar with the monitoring in their cal level, then just do your surround thing.

Keep your upmixers "stereo compatibility" safety switch in - and the down mix of your umpire should be identical to the stereo mix you have already made. It's hard to go wrong unless you are silly. Keep the dialogue up the middle, lay off panning dialogue (unless you are really sure of what you are doing).
It will take a while to get used to real centre speaker rather than phantom centre. Do not go bonkers with surround. Too many mixers get carried away in the play pen at first and it can get very jarring. Personally I mono up any sync reverbs. Its a pet hate to have stereo LR reverb on a mono centre source, it makes your adr stick out like a sore thumb.

Keep an eye out on your loudness and you will be fine.

re line=up. Many schools of thought here. The ratio of LCR to LsRs should be set up by the studio. Many mix at 79 for TV, many others at 82. Do what works for you. Get some Dolby pink noise and pop it in your session and take along an SPL meter so you know where you are. Ultimately though, mix at your chosen cal, but don't be too dynamic though just because the screen is bigger. Many fall foul of that.

Have fun.

Just don't overuse the LFE. I often route some very low level signal to prevent "nothing in the LFE" fails. Not that there is a requirement to use LFE when mixing period drama, but the tech report morons don't get that unless you SPELL IT OUT for the hard of thinking.