Hi Gilmore Girl,
I'm glad I could help!
I think the 13" rMBP would be plenty powerful to work with a large LR catalog, even as big as yours. Mine is only about 10,000 so I can't say I have personal experience with a catalog so big, just general computer knowledge. Anyway, yes, I do edit on a Windows machine primarily, but its mainly for convenience. Its setup at a nice desk with a comfy chair and huge monitors. Its a very nice place to game, and edit photos. I would be happy to use just the 13" rMBP if that was my only computer. Anyway, I think the power of the 2.6GHz would be plenty. I wouldn't worry about that. I think LR will be able to render large libraries well, and it will be able to perform edits on large photos well too. I will admit that it wont be as fast as the i7, but the difference is very small, and certainly not worth that much money (although this is a personal thing, someone with more money than me might disagree

)
You are right about being able to install LR on both Windows and Mac with one license. I plan to do the same. I wish they'd allow that with Photoshop too, but alas, my Photoshop is Windows only, like yours.
You asked if the fact that I have a Windows desktop influenced my decision on my choice of rMBP. The truth is, I have multiple computers, including a 15" rMBP that my job assigned me last year. So its hard to say. However, I thought about it, and if I had to choose only one computer, it would be the 13" rMBP. It is the perfect combination of portability, power, and battery life. You can hook it to a couple big ass monitors and now its a perfect desktop. I really think its just about the perfect balance for a laptop. The only thing I'd like better were if the effective resolution were higher. Not a big deal though cause I will run it at the top scaled resolution of 1680x1050, just like I do with my 15" which I run at 1920x1200. More screen real estate is better for coding (or photo editing probably).
One more important point. Please forgive me if you already know this, but... I hope you have a good backup plan for your photo library. You should keep a copy of your Lightroom library files and all your photos on an external drive. You should keep ANOTHER copy on another external drive which you let your friend hang on to. Just keeping your backup on an external drive on the desk isn't good enough. A fire or theft would cause your photos to be gone forever!
Anyway, I hope this helps