I haven't been on your ship but on other mid-size ships in the fleet (as well as some others) so only can speak to those:
Yes on the traditional formal nights the cruisers really do dress up (about 60% best guess) and usually go to the Captains greeting prior to dinner, the sequins and jewels on the ladies are blinding. A man can get away with a sports jacket if just doing dinner but usually a tux if going to the pre affair. My son has done it both ways with renting a tux for the cruise which was quite reasonable, fit well and looked good. I myself have never done the full dress up but a skirt, nice sundress with jacket or paloozo type pants will work. If you do go all out, it is a good opportunity to get professional portraits done at a reasonable cost.
From the video of your ship it looks like the same gym - decent and free. The spa is an extra cost, the last one I was on had just been revamped with a huge whirlpool compared to your ships (via online video). I believe it worked out to about $10 a day (you have to buy for the full time though) but we got it the first day when the 50% off special was on so if that is something you are looking for then you should go there upon boarding and sign up as they also limit the number of people they allow and the price goes up. The sauna/steamrooms had in the past been used by my son without purchasing the spa package but not sure if that works anymore because they gave us a different key to access the spa area this past summer which is also where the lockers, sauna and steamrooms were located. The private relaxation area (outside with a pool) I believe was $5 - 10 a day (again you have to purchase it for the full length of the cruise) I have never bothered to use it because I only go on cruises with limited at sea days and therefore was off the ship during the day so it didn't make sense. Also since HAL usually has a minimum number of kids on board compared to some other lines and they have a seperate area for them anyway you can usually find quiet corners without having to pay extra.
I have gone on HAL pre and post Carnival did not notice a change in the service (which has always been excellent) but did notice that there were a few more "extra" costs which use to be freebees.
The tradition midnight buffet is actually at 11:00 (sometimes 10:30 so check your daily HAL) although the selection isn't huge the quality is very good and there is usually pizza and some desserts until the wee hours, as well you can order room service 24 hrs a day with a good selection and it is one of the few lines where you can still get a full breakfast cooked to order brought to your room.
As to the after dinner entertainment I think it is hit or miss (my mother in her 70's always loved it), the brought in talent has always been excellent but the shows put on by the onboard staff (musicals) tend to be things from 30's to 50's although a couple of years ago they did a Grease take off which to them was very modern so ...... mind you they are very talented singers and dancers so if you like classics then you are ok there, plus they do have piano bars and movies if you aren't into those.
I have always found HAL to be a great line, partly I believe because of the larger portion of an older crowd which tends to demand a better level of service, are polite rather than pushy or snobby and have never witness a scene or obnoxious drunk which you can find on some other lines. Have a great time!