Hi sorry for the late reply. I'm not an expert here but I think I can help you with a couple of things:

I installed HeliBoard_1.2-release.apk

Then you're fine. That's the app intended for release. I assume you got the APK from the developers GitHub? I use F-Droid so there's only option available.

It's giving me suggestions, but no corrections

You could download that dictionary file that you linked, but I believe that is not necessary if your Android System language is set to English and you're typing in English too, it should be able to use the System internal dictionary. In my case I have 4 different languages configured in Heliboard and I downloaded dictionaries 3 of them, but I didn't need to download the one for my system language, it just uses the internal dictionary. For the other 3 languages: I downloaded their dictionary files, saved them in a safe location in my phone (I have a folder for apps Settings and backups, and I created a Heliboard subfolder where I keep my dictionaries and the glide typing library)

Regarding my old issue with changing languages and having the spell check change at the same time: I fixed that by changing the Spell checker app in Android's System Settings. I found out about this reading a related GitHub issue and I suggested to the developer to add this information to the FAQs, which they quickly did (check the section "Spell checker is not checking all languages in multilingual typing"). You can also activate the Globe icon to Switch languages under Heliboard's Settings: "Preferences/Additional Keys/Language Switch keys", but I personally prefer being rid of it and just sliding on the spacebar