Bird flu (H1N5) is not yet in Australia and you'll know when it is because it's not gonna be great for our wildlife - it will hit chickens or migratory species first though, IM O. And magpies are 'catholic' feeders; as long as you aren't going overboard or being silly and feeding them rubbish they will be fine with a little treat. And good enough hygiene is ok - even with your and Melbcat's health concerns.
Can lightning strike - yes. Are you significantly increasing that risk by carefully connecting with wildlife - no.
I put out water on a hot day but when it's not summer there's tons around.
source: I'm not a biologist but I work with and hang out with birdos. Also have experience living with immunocompromised peeps so I do understand
You are doing amazing π€