How to Constructively Deal with Email Invitations to Scam Conferences or Journals?
I am going to focus on your point 1:

What arguments can I make to my academic institution's IT department that blocking these emails should be a priority?

Please don't.
If you manage to convince your IT department to make it a priority to block such emails, then it's quite likely that there will be false positives across your institute. You say that it is straightforward to identify these scam emails, and I have no doubt that as a human, indeed, you can. But there will also be humans writing invite emails on behalf of legit conferences or journals. Those humans may not have English as their first language. They may inadvertently use turns of phrase that you find characteristic of spam invites. As a consequence, some legit invites will be discarded along with the spam invites. The cost of these discards is asymmetrical: the cost of manually discarding spam is a slight annoyance, but the cost of having a legit invite discarded could be a missed career opportunity.
Ask yourself: how many colleagues missing out on legit invitations is it worth for you to no longer have to manually discard the spam invitations?