I like approach B with the default value=1 instead of 0. (...)
Yep. That's something shahzadafzal suggested, too (approach B with "1" instead of "0" as the default). I do like that approach, but it relies exclusively on JavaScript, and leaves the no-JS folks with basically the same problem as before (only now, instead of 0-based mistakes being easy to make, 1-based mistakes would be easy to make: accidentally sending 20 merits when you meant to send 2 would just become accidentally sending 21 merits). Catering to people that have JavaScript disabled is getting a bit silly these days, but it's still the way I lean when it's easy to do so. I also think that having a sendable default value will likely skew the statistics (that is, people will probably send exactly the default amount more often than they would have if they had to type something into an empty field).
I don't like "1-50" as a placeholder. If you know nothing about merits, that doesn't really help you. 1 minus 50 so... -49 ??
Hmm. You and Loyce can duke it out. "0" as a placeholder is the least distruptive change, I suppose (it's what people have gotten used to seeing in that field, anyway).
I'd say leave it empty.
Even compared to a "0" placeholder?
Ensure the field is focused automatically so that the user can just type in the number and hit enter ("autofocus" attribute IIRC).
You didn't read the P.S. in my previous post, did you?
