Discalimer, I don't own a single 10mm or .40 gun...
1) Yes you can absolutely use a .45ACP suppressor. I think you'll find your selection of .40 suppressors really limited. I don't think there will be much dB reduction difference, as there's not a big difference in dB reduction with 9mm in a .45 suppressor (partially due to the volume of .45 suppressors).
2) Personal preference. I prefer 3 lug, as it doesn't back out, it's faster and easier to attach and detach. Threaded will be simpler. Depending on whatever your gun is (clone vs. actual HK), you might have fitment issues with the 3 lug. Note that .40 and 10mm MP5s have a different size 3 lug than 9mm.
3) Yikes, a dicey topic. If you're shooting suppressed, I think subsonic 9mm is easier to find. But again, see the initial disclaimer of I don't own .40.
4) I still see them on their website?