1) When I click the validate button, it indeed validates brackets, but appears to now be a "toggle" rather than a button
I have not been able to reproduce an issue with this. If I click the button, it aligns text, checks for invalid brackets, and so on. If I click it again, it does the same thing. Can you describe your issue a little more?
Update: if you are referring to the appearance of the button, ie. it is highlighted/unhighlighted, I see this as well. This seems to be standard dialog behaviour when a different button from the default is clicked. It has behaved this way at least as far back as v6.35. So far, I cannot see a way around it.
2) Replace all does not work....
As far as I can tell, this is working correctly here. If I paste your text into the editor, move the cursor to the top, and "replace all", it works as expected. Is it possible that you have left the cursor at the bottom of the page? It will only replace from the cursor onwards. If it cannot find any more items, it beeps to indicate it has reached the end of the file. mIRC is behaving in the same way as Notepad++ (that was the purpose of the change in v7.73, Item 18, to make the dialog work like most applications - in this case, I often use NotePad++, which is probably one of the most widely used text editors, so I modelled it on that). That said, Notepad++ has a "wrap around" option in the find/replace dialog, which mIRC does not have. I may add this in future.
Update: I have found an issue which sounds like the one you were reporting. This is not related to "Replace all" but to clicking the Find button to find an item and then clicking the replace button. It skips the first replace in that case. This has been fixed for the next beta.