Can anyone please recommend a more standard screw that would replace Apple's P6/TS5/PL5/1.5mm Pentalobe screw (the flower shaped one), which wouldn't strip or damage the existing thread in the casing? (Ideally you will have tried this and been successful, rather than just guessing. 
)
The screw in question secures the battery in a 2009 15" MacBook Pro (non-retina). The battery is held by 2 of these custom P6 Pentalobe screws - AFAIK this is the only place Apple ever used this size of Pentalobe screw, so even finding a screwdriver has proved exhausting and expensive. So frikkin' annoying, Apple.
)The screw in question secures the battery in a 2009 15" MacBook Pro (non-retina). The battery is held by 2 of these custom P6 Pentalobe screws - AFAIK this is the only place Apple ever used this size of Pentalobe screw, so even finding a screwdriver has proved exhausting and expensive. So frikkin' annoying, Apple.