I've been using IL  5+ years and have about 25% of clients willing/capable of using it. I can't/don't want to figure out/utilize the intended Intuit workflow... it's too convoluted for me... . I prefer to use my own filing system and work flow...(what I like best is I can easily copy all their files into my client folders at once, and most of my clients  correctly label their files)....

I don't get real fancy w/ the clients that use it....   I send them my own fillable organizer and a note....  "drag and drop your stuff into the portal....   be sure the dropdown is for TY 2024 (fingers crossed).... Let me know when you're done  so I can start preparing...  I upload the finished returns and send them a canned notification... (wish I could copy myself into the notifications... but I'm just dreaming out loud...)

Your questions 1-4 are good thoughts... but don't hold your breath...  seems like the ProConnect version  functionality is more robust and obviously the Intuit priority... just consider yourself a poor stepchild... (#4...  yes you have to manually update and change email addresses) Regarding #5...  LOL.   (your might want to put it in the suggestion box w/ your other x-mas wishes)...

<< When Intuit Link works my clients love it but when something happens that I can't fix the clients get frustrated when we have to go back to insecure email attachments.>>   Yup.... 

For me, I've learned not to set high expectations, so I'm not easily disappointed... .