Internet Explorer has less than a 2% market share in the US, so under normal circumstances (given the sacrifices you’d have to make everywhere else) not bothering to support is is often a sensible decision. When it’s an access to healthcare issue though you can’t really exclude 2% of users.

That said, building something that works well on mobile (re your point b) but also works on internet explorer is a pretty tough ask. No reason you couldn’t build a basic bitches version of the site that works on old devices, then route traffic these based on user agent.

Idk, points a through c are basically already solved problems. I’d be disappointed to see problems like that if I was spending $20,000 let alone millions. Issue d though does feel like the sort of thing that you didn’t know was on the horizon up front, would be a pretty major rebuild to add in later on.