1. If I understood you correctly, you had set the WP server time to UTC and therefore the airdrops all run on UTC time as expected. Does this mean that when you switch the site-wide timezone setting to your timezone UTC+2, the airdrops now run on UTC+2? (This is how the plugin was designed and tested.)
2. For this extension I have implemented a separate scheduler that is independent from all the other cron jobs. If you create an airdrop that is applicable in the past, then it will run once every minute until it catches up, but is guaranteed to eventually pay out all the airdrops corresponding to the time period you specified. I will make this clearer in the product description. Thank you.
3. The SQL problem you describe is not related to this plugin. Instead it seems different DB tables are created with different collations, which is unusual. Did you manipulate tables recently, such as importing or exporting tables? You should make sure that all WordPress tables have the same collation. This is the same issue reported here: https://wpml.org/forums/topic/wp4-6-0-illegal-mix-of-collations-when-using-utf8mb4_unicode_520_ci/ and here: https://www.satollo.net/query-error-illegal-mix-of-collations-utf8mb4_unicode_ciimplicit-and-utf8mb4_unicode_520_ciimplicit
The solution is to force all your tables to have the same collation, for example:
To do this automatically you can try this tool (I haven’t tested it) https://wordpress.org/plugins/database-collation-fix/ or you can manually modify the collations from your SQL console or phpMySQL.
Whatever you do, back up your database first.