HD calling is just what it sounds like - High Definition calling, better reception & sound quality. I do believe you'll still be able to connect to the internet while on a call since it's not a special feature.
Nope
The carrier version software includes what is needed to operate the carrier Wifi calling and other advanced calling features. If you buy an unlocked version you don't get the pre loaded software. Many consider this bloat(and yea there's alot that really isn't necessary unless you actually use it - if you don't it is bloat)but it also includes what is used for all enhanced LTE features including but not limited to WiFi calling, video calling, HD voice, number sync and advanced messaging. So you will lose out on all enhanced LTE features since you can't download this software elsewhere. I've been able to actually delete some of the pre loaded bloat and there wasn't as much on the S9 as there has been in the past.
If you don't use these feature the unlocked version will work fine but If you do its worth the 70$ between 789$ & 719$ at full price.
I use many of the features like WiFi calling so I went with the carrier version. I definitely think its a phone worth getting.