1. Is there a way to make the orgams more powerful and last longer?
Yes, you need a vaginal orgasm. This requires a simultaneous stimulation on your clitoris, G-spot and Epicenter.
http://action.love/love/method.htm
Clitoral orgasm is weak. That is why you have mastrurbate 2 or 3 times to burn your testosterone and discharge your sexual energy.
2. Is it true that if you use them it will become harder for your partner to bring you to orgasm? Your partner's penis is very difficult to compete with the vibrator in term of stimulation speed and pressure. However, a long-term use of the vibrator will damage or numb your sexual nerves. My suggestion to overcome this problem is given in
http://action.love/cases/case8848.htm
3. whether there is any such thing as overmasturbation for a female?
Yes, if it is the Level-6 or -7 vaginal orgasm, resulting in orgasmic or post-orgasmic pains/cramps in the low abdomen perienum, grions or the tail bone and anal area, but you can have a lot of clitoral orgasms without worrying about over-masturbation