Apart from what pooya87 has already said, there are some edge cases which not many people know about. For example, if you lose your channel database, you need to initiate DLP (data-loss protection) in order to get your funds back. You can do it as long as you have a backup file which was generated after you have opened all of your channels. That file contains all the necessary information you need to reestablish the connection to your peers and ask them to close your channels. Here are the possible scenarios:
1) Everything goes well and all of your peers immediately broadcast the latest commitment transaction of each channel. You will very likely get your funds back after 144 blocks have been mined.
2) Some peer takes the risk and closes the channel with an old commitment transaction. You cannot broadcast a penalty transaction because you lost all of your data.
3) Some peer is offline and cannot respond to your request. Unless they come back online, the funds are locked up in that channel which means that they are lost in a limbo.
That's why keysend should be required by the campaign manager. Unfortunately, it looks like it is supported only by LND and c-lightning so the participants would have to run either of these implementations 24/7.
Each invoice contains a hashed payment preimage. In order to claim the payment (or HTLC to be more precise), the preimage needs to be revealed to the payer. Thus, the campaign manager could prove that the payment has been claimed by revealing both the invoice and the payment preimage.
Last week, the participants got paid over 0.19 BTC in total. Currently, most nodes reject channels larger than ~0.1677 BTC so CM would have to either open multiple channels or a single channel to a large HUB which supports Wumbo.