Thank you VERY much for your in depth analysis. It helped me import almost all cards sofar!
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So with all the under-the-hood background knowledge out of the way, a couple questions: Does DelverLens a) export a CSV directly, or b) just present you with a bunch of text that you have to copy-and-paste into a CSV file yourself? If it does a) and exports a CSV directly, have you tried importing using that file without ever opening and saving it in Excel? If b) and you have to save it as a CSV file yourself, then try saving it in a basic text editor like Notepad or something.Basically, either DelverLens isn't giving you a useful export, or something in your process steps altered it to its current form. We need to figure out which so this doesn't happen to you in the future.
I'll show the flow in Delver through a couple of screenshots (thumbnails, click to view). When you press the Export icon, the following pop-up appears:
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In my case, I chose 'Create CSV file'.
I then get the option to add columns, choose the seperator or chose a preset. I chose the latter::fill(white):strip_exif()/f/image/UQUfU0h8LmQtZUEDYCFB8shF.png?f=user_large)
And then chose 'CSV - Deckbox - no tradelist'::fill(white):strip_exif()/f/image/hqhFN6A9GKn29tAsQh4k42fM.png?f=user_large)
The file is then being prepared and I can choose to either share or dowload it. What I did earlier was share it to my Google Drive, inspect through Excel then save and upload, resulting in those 5000+ errors. Upon reading your reply, I decided to instead download the file to my phone, and upload it directly to Deckbox. This resulted in only 40 errors::fill(white):strip_exif()/f/image/1tSlZSrL0qVDtVFGky9XnGf0.png?f=user_large)
I re-uploaded the valid rows, and now I have 5472 cards in my Inventory. Great stuff! You were right, Excel messed up the file.
However, just like in that other forum post you linked to, there were a bunch of mismatching edition names. I went ahead and helped you out by listing all of the ones you need to change and what to change them to.
Normal sets:
DELVERLENS NAME DROPBOX NAME Commander 2011 Commander Duel Decks Anthology: Divine vs. Demonic Duel Decks Anthology, Divine vs. Demonic Duel Decks Anthology: Jace vs. Chandra Duel Decks Anthology, Jace vs. Chandra Foreign Black Border Foreign Black Bordered Fourth Edition Foreign Black Border Fourth Edition: Black Bordered GRN Guild Kit Guilds of Ravnica Guild Kit Intl. Collectors’ Edition International Collectors' Edition Modern Masters Edition Modern Masters Return to Ravnica Tokens* Extras: Return to Ravnica Rivals Quick Start Set Introductory Two-Player Set Summer Magic / Edgar Summer Magic Time Spiral Timeshifted Time Spiral "Timeshifted" * On this one, the card name also had to change by removing the word "token".I recommmend double-checking all the ones marked as FBB, Summer Magic, Intro 2-Player, and Int'l Collectors. According to the guy from the other forum post, DelverLens has a harder time identifying those older cards, and all the ones he had that it marked as Summer Magic and the like ended up being normal versions of the card.
Promo sets:
CARD NAME DELVERLENS SET NAME DECKBOX SET NAME Arbiter of the Ideal Born of the Gods Promos Prerelease Events Temur War Shaman Fate Reforged Promos Intro Pack Alternate Art Sin Collector Friday Night Magic 2013 Friday Night Magic Zameck Guildmage Gatecrash Promos Magic Game Day Cards Skarrg Goliath Gatecrash Promos Launch Parties Giant Growth Junior Super Series Super Series Xathrid Gorgon Magic 2013 Promos Prerelease Events Jeering Instigator Ugin's Fate Ugin's Fate Promos Serra Angel Wizards of the Coast Online Store WotC Online Store Sprouting Thrinax Wizards Play Network 2008 WPN/Gateway Master's Call Wizards Play Network 2011 WPN/GatewayPromos are the trickiest, because there is no standardized system for naming promo editions, nor even for grouping promos into editions. Notice how the DelverLens edition "Gatecrash Promos" turned into two different Deckbox editions, while two different DelverLens editions turned into the Deckbox edition "Prerelease Events". With promos, you just have to check them card-by-card on Deckbox. I'm not looking at all your cards, obviously, so double check that these Deckbox versions do indeed line up with what you own.
Indeed, this is what's left to do. The 40 invalid rows fail on exactly this point. It's 'only' 40 rows, so I could add them manually, but I'll try to do it through the .csv instead. Might learn something there 
Thanks again!
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