How many people can use the same card on Google Wallet?
Here's my direct experience with something similar on a much smaller scale:

My girlfriend and I setup a joint Google account so we could share purchased apps. I added to it a card of mine that was already on my own Wallet account. The new account was IMMEDIATELY frozen and I couldn't make any purchases. After digging into it, I was told that if there's any suspicious activity, especially on a new account, including adding a card that already exists on another account, they automatically freeze it for an unspecified period of time.

In my case, about 48 hours later it unfroze.

I realize this doesn't confirm what would happen if you did this for 100+ accounts, but from experience they did react rather dramatically to it being added to only ONE additional account. I suspect I was unfrozen on goodwill as the account didn't exhibit any other suspicious behavior. The card being added over and over to dozens of accounts would likely trip higher-level oversight and lead to a problem.

One note: I've actually had good experience with Google's phone support when resolving a Wallet-related problem with the Play Store. The person I spoke to was helpful and was able to contact a higher-up for policy information, and provided me with an accurate solution. Supposedly this number is a direct line to their support. I'd recommend getting an official statement from them before making any major decisions.