1. 'Happiness' is subjective. For someone happiness is to have many kids, for another one it's a problem and that's why he/she is childfree.
Let's say you need X-happiness, if your X-happiness = A + B + C, then you have to achieve A, B and C in order to achieve your X-happiness. And if you need Z-happiness and it's: Z-happiness = A + B, then you have to achieve A and B...
To achieve happiness for sure, you need to have/to choose simpler, easier happiness. It's easier to be happy, if you need something easy to achieve than if you need something hard to achieve. For example, if you need to read a new book and this makes you really happy, you'll have your happiness easier, than another person, who is happy only if he/she got a few million dollars.
2. 'Success'. There are different kinds of success -- success is to learn the alphabet, success is to earn USD 1000, success is to make a website and so on.
You need resources to achieve success like: knowledge, will, skills,... in many cases -- money, relationships...
If you choose easier goals, then your success achievement will be better.
3. About the goals. I think the answer is the same as the one above (about the success).
Conclusion:
easy goals = easy success = easy happiness.