I haven't had many issues with scanning off mobile devices at multiple Tessitura venues. Best practice is to inform patrons to scan off the actual email (instead of taking a screenshot) that way you can resize the barcode if necessary. Also make sure the brightness is turned up on their phones! You can include these instructions in your confirmation emails or with signage at the doors. My experience is that patrons who choose the PAH option are often well-versed in the process.
When training ushers on handling patrons phones I would just recommend they give verbal instructions to the patron (open the PDF, resize the barcode, turn up the brightness etc) and only handle the patron's phone themselves, if they're comfortable, as a last resort and after asking the patron's permission. Mobile devices carry a lot of personal information, so you don't want to seem like you're infringing on personal property!Good luck!