1. More weight or more difficult exercises. More food.
2. Maybe? Mostly hardgainers are either not lifting heavy enough or not eating big enough. Did your parents every actually try to gain mass? How do you know they are hardgainers? Physical size has a lot to do with nutrition while growing up. Your parents may have just received less-than-adequate nutrition while they were growing up, for whatever reason.
3. I don't know, it depends on how tall you are, how much you eat, and how big you lift. Yes, it will slow down. Set other goals besides a number on the scale. Pistol squats, 1-arm pushups, and 1-arm pullups are a good start. How tall are you?
4. Muscle takes a long time to lose, but yes, if you don't stimulate your muscles, they will waste away. Set other goals besides a number on the scale.
Bodyweight can give you what you want too, quite probably better than the equipment you are using. You are not lifting enough weight. A sandbag can be made cheaply and is very effective. A man should be looking to squat at least 1.5-2 times his own bodyweight. 50 lbs of books in a suitcase is not going to do it for you.
A strength-based bodyweight program (Al Kavadlo, Convict Conditioning, Building the Gymnastic Body, You Are Your Own Gym, etc) will help you out here as well. Combining one of these programs with the weights that you have may be useful.
Mostly, it seems you need a strength program, you don't know what one is, and you are trying to make one up. Other people have done the work for you, there is no reason for you to make up a program. I don't care what it is, but find a strength program, start at the beginning, and follow it step by step. There's someone that even has a Sandbag Fitness program that posts on the forums here. Follow a program.
For example: Situps are a pretty useless exercise, and 7 pounds is nowhere near enough weight to stimulate the muscle that you want. They don't build actual strength or muscles, except for the little useless pretty ones on top.
"Pretty much a pullup for each arm" is not the same as a pullup, which actually requires you to move your body through space, stabilizing yourself with some really intense abdominal action.
Finally, as I said earlier, find goals that are not based on the scale, something you want to do. Staring at the scale is not good for people losing weight, nor is it good for people trying to gain weight. Train for getting stronger. Eat healthily and sleep well for proper recovery. This will build muscle. Building muscle will make you weigh more. Some people want to do things, some people are motivated by numbers they can lift, or speeds they can run, or obstacles they can overcome. You have to find what motivates you- what do you want to do that you can't now, or could do better if you were stronger?