Ok, this is bizarre. I don't know if it is encouraging or not..we will see.
First, on my other machine, I did a fresh install. (i had issue a while ago with it and hadn't used it in some time). Anyway, I was having a terrible time getting BTS to run at all. I knew the install was fine, so I check around Steam to see if there was anything unusual. I think fairly recently Steam as added a property setting on the General tab dealing with VR. Anyway, I unchecked it and the game ran fine. Now, this likely has nothing to do with the issue at hand (maybe), but may be related to some of the issues you had running the game initially.
So, first of all:
1) Right click on BTS in the Steam Libary
2) Select Properties
3) On the General tab uncheck the two checkboxes you see (Steam Overlay, and Steam Theatre/VR thingamajig) - they are unnecessary, and the overlay thing is annoying anyway
I did not encounter that issue on my other machine, but I had installed the steam version on that months ago, so maybe that setting was added sometime later and did not effect it. Anyway, it was causing the game to crash when trying to change graphic settings.
So, now what is bizarre, is that after installing BTS and converting to the Beta, I noticed the same thing as you. The DLL is 4834 which is the same size as the non-steam version. The DLL of the regular (non-beta) steam install is 4828, and that is what I have on my other machine. So I don't know if something change with the Steam version since I installed on Laptop A, or if I just did something I don't remember way back when.
Bottom line though is that on Laptop B, I did not have to do anything. I loaded that save fine after installing BUFFY.
Well, actually I did do something which I would like you to try. It's actually a little quirk some other games have on Steam too.
Now we can try this pretty simple thing, but if it doesn't work this time, we might try a fresh install again and then these steps.
So try this:
1) Right click on BTS in the Steam Library
2) Go to Betas tab
3) From drop down select "None - Opt out of all beta programs"
4) Steam will perform an auto/install again of some BTS assets to put it back to the regular steam version
5) Once complete go back to the Betas tab
6) From drop down select "original_release_unsupported"
7) Again, Steam will auto-install some bts assets to get it back to the original non-steam version
8) Load the game once and exit
9) Go back to BTS properties in Steam Library and select the "Local Files" tab
10) Click "Verify Integrity of Games files" (this may take a few minutes based on your machine)
11) Once complete you should see a message something like "11 files failed to verify - Steam is redownloading those files" and a quick download will take place
12) First check the Asset folder and note if the DLL is 4834
13) Load Buffy and try opening the save
Note: I don't know if all of what I did was necessary for you, but I was just retracing the steps I did today, so might as well match up. I don't know if you have jacked around the with game some since yesterday that might impact the results. If so, and this does not work the first time, then we can try a fresh install. Perform the steps above first before anything, and then go back and just do the HOF steps I provided in the link above (admin rights, security, read only stuff) - nothing else. Ignore the DLL stuff I posted before for now.