What should I do about water mixed with a small amount of menstrual blood? Is it obligatory to perform istinja' after menstrual blood? And is it sufficient to use only paper tissues for istinja' after urinating and defecating during menstruation?

It is clear from your question that you are afflicted with some form of waswasah (obsessive whispering). We advise you to turn away from these waswasah and not pay attention to them. Know that a small amount of menstrual blood is forgiven, according to the preponderant opinion. Since this issue is subject to differing opinions, and you are afflicted with waswasah, you may adopt the easiest of the opinions. Therefore, the water used for istinja’ (washing after relieving oneself) that may be mixed with a small amount of menstrual blood, and sweat mixed with a small amount of menstrual blood, are forgiven. The matter is broad. It is permissible to perform istijmar (cleaning oneself with dry material) using paper tissues and the like, whether during menstruation or at other times, provided certain conditions, explained in many fatwas, are met.