Praise be to Allah.
It is very clear that you are suffering from waswaas (compulsive intrusive thoughts) concerning matters of purification, because you are asking about ghusl for entering Islam even though you are already Muslim, praise be to Allah. Waswaas is a chronic problem; we ask Allah, may He be exalted, to heal you from it.
Al-‘Allaamah Ibn Hajar al-Haytami was asked about the disease of waswasah: is there any remedy for it? He replied: There is an effective remedy for it, which is to ignore it altogether.
If one is uncertain and has doubts about something, then if he does not pay any attention to it, the state of uncertainty will not continue; rather it will disappear, as some fortunate people have experienced. But for the one who pays attention to his uncertainty and doubt and acts upon it, it will continue to get worse until it drives him crazy and even worse than that, as we have seen in the case of many people who suffered from that and paid attention to it and to the devil who causes it, concerning whom the Prophet (blessings and peace of Allah be upon him) warned us when he said: “Beware of the one who causes waswaas regarding wudoo’ and ghusl, who is called al-walhaan [the one who causes waste], because of what that leads to of wasting time and going to extremes in washing.” There is a report in as-Saheehayn which supports what I have mentioned, which is that the one who suffers from waswasah should seek refuge with Allah and stop these thoughts.
Think about this effective remedy which was taught to his ummah by the one who did not speak on the basis of his own whims and desires, and understand that the one who is deprived of this remedy is deprived of much good. For waswasah comes from the Shaytaan, according to scholarly consensus, and the only aim of the accursed one is to cause the believer to fall into confusion and misguidance, make his life miserable, and cause him distress and frustration to the point that he causes him to leave Islam, without him realizing that the Shaytaan is an enemy to us, so we should take him as an enemy.
End quote from al-Fataawa al-Fiqhiyyah al-Kubra (1/149).
You should understand that experiencing compulsive intrusive thoughts (waswasah) is a sickness like any other sickness, and it has a well-known remedy, and may also be treated with behavioural therapy. We think that combining the two types of remedy is most beneficial for the one who is sick, and is most likely to bring about healing. So if you consult a doctor who specializes in mental illness, that will be beneficial for you, by Allah’s leave.
We have explained previously that waswaas [intrusive thoughts; whispers from the Shaytaan] may be dispelled by seeking refuge with Allah and stopping those thoughts. See that in the answer to question no. 20159 .
With regard to your uncertainty as to whether you were awake when the maniy was emitted, that does not mean that you must do ghusl, because uncertainty does not mean anything.
As for the one who wakes up from sleep and notices wetness on his garment, one of three scenarios must apply, as has been explained previously in the answer to question no. 22705 .
We do not think that you should do ghusl in this case on the grounds of erring on the side of caution or being on the safe side, because erring on the side of caution is only prescribed for the one who is not affected by waswaas [intrusive thought]. As for the one who is affected by waswaas, if he does something to be on the safe side, that will lead to exacerbation of the waswaas and will cause a great deal of trouble for him; in fact it might lead to a great deal of mischief and trouble, as is well known in the case of those who are affected by waswaas. We ask Allah to keep us safe and sound.
Combining the intentions of doing ghusl for janaabah and ghusl to purify oneself following menses is permissible. Ibn Qudaamah said in al-Mughni (1/162): If there are two reasons that make ghusl obligatory, such as menses and janaabah, or if the “two circumcised parts” met and ejaculation occurred, and the individual intends both when purifying himself, it is valid for both. This was stated by most of the scholars, including ‘Ata’, Abu’z-Zinnaad, Rabee‘ah, Maalik, ash-Shaafa‘i, Ishaaq and ashaab ar-ra’y. End quote.
With regard to ghusl for entering Islam, it is not prescribed in your case at all, because you – by Allah’s grace – are already Muslim and have not left the religion of Islam. Rather the Shaytaan has made you think that in order to cause you trouble and hardship, and to make you resent religion. So ignore this waswaas, may Allah have mercy on you, because it leads to evil consequences.
And Allah knows best.