One small observation I have about the web version is that in Step 1 the "words you've added" section felt a little redundant. The user already sees a nice preview of the Bingo card as it is being updated. So each word the user adds to the card eventually appears on the screen three times. Since the Bingo card updates lag the "words you've added" updates the resulting user experience is a little confusing. But functionally everything I tried in this step worked, so good job on the word-lists controller - methods new (post) and edit (put) handled my mistakes (e.g., too few words) flawlessly.
Steps 2 and 3 in the web version made sense and the resulting pdf I downloaded looked great. You make a decent case for registering during step 3. At first I didn't understand the line "use 604 bingo activities any time you want to" but I soon realized that's a reference to your library of premade cards, which I tried and thought was a super feature.
When you introduce this web version into the home screen layout, you might want to pump it as the #1 option above the PC and Mac versions. Otherwise it might be too confusing because while the copy on your site is well-written it's pretty verbose. I would want to abstract out some of your sales pitch and whittle down the benefits - although this is maybe personal taste. As you iterate you might try to get the "a picture is worth a thousand words" screenshots higher in the flow. Also you should update those screenshots because the end result today looks significantly better than your images. That may be affecting your conversion.
As I understand you, the sole reason for purchasing a license is to remove the watermark and footnote. I notice however that neither the watermark nor the footnote seeps into the Bingo card. Since people are already using a scissors or a paper cutter to trim the (multiple cards per page) hardcopy it seems trivial for them to also crop away your branding. Assuming it's technically feasible I might inconvenience the nonpayers a bit more by positioning my watermark directly on the cards. But I see you have 1,800 paying customers - so it is me who should be asking you for suggestions.