My experience in schools has not been around subsidies for parents for BYOD but to your other questions around short term replacements:
- I know many schools buy ex-lease TELA devices to keep as BYOD spares for students that either can't afford one at all, or as short term replacements.
- Many schools have tried to use the Library as the central point for lending short term replacements (e.g. checking them out in same loaning system as library books with barcode / QR codes on the laptop lid).
- Some schools have asked for their community to donate devices (usually with some parameters around them in terms of age/specs/battery life etc) that can be added to the pool of available devices. A quick SSD swap out of a 1TB HDD in a donated device should make it very functional.
- Most schools that have short term loans have them in some capacity managed by an MDM depending on the OS of the device so it's very easy to reset as/when required.
- Most short term loans from library are day loans only (unless prior agreement) and no power cord so the the device doesn't disappear at the end of the day and not be seen again.
In my experience in schools I've managed these programs, students always treat their own device better than the school owned devices. The low level damage inflicted on school owned devices from studnets whilst waiting for them to boot or bored (picking at edges of laptop case, demonstrating their creativity by re-arranging the keyboard keys into four letter words etc) all disappears when it's their personal device.....
Hope this helps,