DO keep in mind that the more followers an account has, the lower your engagement rate will become. There is only so much you can do. Although a 25% engagement rate on an 5k account is quite good, a 10% engagement rate on an account with 50k followers is equally the same. (This is just an example and is not accurate).
If you only have a few accounts, then YES reply to any comments that you can reply to. This will make each person who commented feel special. This also increases the user you replied to may comment back (increasing your engagement). In addition others will get to see your personality, and may choose to also comment or at least have a more emotional connection to you & your account. Instagram loves comments, and the more comments your picture has, the higher chance it will get featured, and the more exposure your account will be granted.
If you have lots of accounts, it will be very difficult to reply to your comments. In this case you may want to only reply to comments on a few of your top accounts to keep them growing fast and maintain a high engagement rate.
The answer to this is very similar to the one previous. The only difference is the fact you are boosting their engagement rate, and the only effect it has on you is that they may be more active than before. I personally would target users who dont interact with my content as much as others do. This would have a larger impact since they can obviously spend more time on my account compared to someone who is already engaging quite a bit.
In any business, you always want to have regular customers, so you provide them with loyalty discounts. With that being said, they can only buy so much since they only make so much/month. So in order to increase your sales more dramatically you want to increase the amount of new customers and encourage them to buy things more often. You need a good balance. So in the sense of social media, you already have your regulars who engage daily as much as they can. If they are doing this continually there is no need to comment on their posts. It would be better to get new followers to do the same. In Instagram's eyes, they prefer the majority engages with your content less often vs a select few who engage often.
If you have hundreds of accounts this will also be very difficult to do.
You can however use the auto comment feature and comment on your followers pictures using emojis. Since its automated, you do not want to appear as a robot with random replies that do not relate to the posted content. So you want to reply with generic replies which will suit nearly any picture. Emojis are the best way to do this.
Lastly, every account is limited to a unknown amount of comments/day. With that being said we want to be as productive as possible with each comment. Your first priority would be to reply to comments on your own pictures. If you have not met your unknown amount of comments that day, you can use the remainder to comment on your followers pictures.