I've worked more on the chronology. I'm lacking the release dates of Pinkie Pie's Special Day and Meet the Ponies. The mlp wiki gives their release years, but since they were shortpacked into toy releases, they don't have the usual thorough date documentation that one might find with TV and movie releases. I usually just give a placeholder date of January 1st if I don't have more precise dates. I'd like to be able to buy the DVDs apart from the toys, since I don't collect toys and since it drives up the price of whatever I'm buying.

The G3.5 webisodes Sweetie Belle's Gumball House Surprise and Pinkie Pie's Ferris Wheel Adventure apparently only appeared online and were never released to DVD. I've seen posts stating that they were included in the playsets, but eBay listings easily confirm that there were no DVD releases included. I'd like to find the Flash files as they appeared on the Hasbro website. They're on YouTube, but raster graphics don't have the same clarity as vector graphics. Again, the release dates are absent apart from the year they were released.

So Many Ways to Play and Over Two Rainbows were originally included as a DVD titled Once Upon a My Little Pony Time. I can't find more than one reference that says the DVD was included in a toy release. If it was, it would have been with a Newborn Cuties playset, not with a pony. I can't find any eBay listings for the DVD, but I have seen images of the outer DVD slipcase; I'm going to assume the DVD does exist. Even if the DVD does exist, it probably isn't going to include the Flash version.

Here's the link to the WIP. https://docs.google.com/spreadsheets/d/167f6JIrnf_aVNVDsWJuY5jOgiIzi5G3LvpkzajSsWoY
The most expensive videos to buy are via iTunes and directly from Shout! Factory. You get the highest resolution from blurays first (or really the original Flash files of anything), iTunes streaming second, DVDs third, and VHS fourth. It would cost about $475 USD to get everything from the list, not including the above mentioned shorts that apparently can't be bought anywhere.

Thanks for all the replies, people. It's way easier doing this with feedback.
A little bit of background in case someone wants to acquire the intellectual property rights to Rescue at Midnight Castle/Firefly's Adventure/My Little Pony.
Children's Home Library was a division of Vestron Video, which was purchased entirely by LIVE Entertainment, which then sold most of it's Vestron properties to Lion's Gate Entertainment. I assume My Little Pony (1984) is one of those properties (especially considering Lionsgate was the distributor of the modern MLP: The Movie). It's out of print.
Some clarification and disambiguation on what I summarized regarding the My Little Pony Special (which was never called the MLP Special). "It originally aired with the single title only, My Little Pony, a la Star Wars, then had a secondary title applied to it when it aired in two parts during syndication: Rescue at Midnight Castle. At this point, Rescue at Midnight Castle was still airing in its original, uncut version. Rescue at Midnight Castle is not to be confused with the later edited release called Rescue from Midnight Castle. There aren't two versions of RaMC--there's just the unedited aired television special which was never seen on VHS called RaMC, and the edited version that later aired and was eventually put on DVD called RfMC. After it had been in syndication for a while, My Little Pony was released to VHS in its original television special length, this time titled Firefly's Adventure. RaMC was never released to VHS. My Little Pony was released to the PAL VHS format for the UK as My Little Pony in Dreamland.
http://mylittlewiki.org/wiki/File:Video14.JPG
In 2003, almost two decades after the original airing of My Little Pony, Disney started airing edited versions of the original series: this particular entry was edited and aired as Rescue from Midnight Castle.
According to http://mylittlewiki.org/wiki/My_Little_Pony_Special, "a scene including the 'Little Piece of Rainbow' song had been edited out."

With regard to getting it backed up and archived, it's probably best to use a US NTSC copy of the tape anyway. Pretty sure transferring something from NTSC to PAL introduces data loss. They're selling for around $25 online. I couldn't find any copies of Firefly's Adventure online. There's a company called digitalfaq that does tape to digital storage transfers. I looked into buying the equipment and doing it myself, but in order to preserve the quality of the media as much as possible it would take about $1000 worth of equipment. I don't have that in my budget right now. I imagine the tape transfer expenses would be pretty affordable if a video transfer company was to be hired, especially for a half-hour tape. That would only produce a personal copy and wouldn't settle the IP rights issue for mass distribution.
http://www.digitalfaq.com/services/video-conversion-streaming.htm

Been in the fandom since January 2011, seen all the FiM episodes, and have browsed random bronarts since 2008-ish. Attendee of Canterlot Gardens 2012 and Cutie Mark Con 2013.