Yeah, it can be done. All these activities take around 1 hour each. I did scuba diving twice in Havelock & it was super fun. You can check this for some info. Most definitely. It is perfect for a honeymoon. You'll love the pristine clear beaches and also a lot of activities that you can do there. You can refer this for some I sights. There's a video you can check to see how beautiful the islands are. https://www.tripoto.com/trip/explore-andaman-in-5-days-5d46b3bc2755a
Yes, you can easily do snorkelling, scuba diving and sea walking in one single day. Snorkelling takes about 15-20 minutes.
Scuba dive requires - 1 to 1.5 hours (considering training time)
Under sea walk takes no longer than 30 minutes.