I happened to pass by these forums after many years of not posting and saw these questions, so I thought I would answer them the best I can.
[quote="Dolezal, post:1, topic:311900"]
Besides obligatory prayer such as the five daily prayers, do Muslims also practice intercessory prayer?
Do Muslims venerate the prophets or other people and ask for their intercession? Do they pray to or venerate the Blessed Virgin?
There is no prayer in the sense of "solat" (the formal prayer which involves bowing and prostrating), however in terms of supplication (du'a) the scholars have differed over the permissibility of asking for intercession through prophets, pious people, etc. It's called "tawassul" and it is a disagreed upon matter in Islam. The more literalist schools reject its application after the death of the Prophet while the main schools accept it, in various degrees (some say the Prophet only as per a hadith whereby others extend it to other pious people). Either way it's not an obligation to make such a supplication.
Does Islam have a concept of mortal sin? If not, what should prevent a Muslim from sinning?
Muslims consider every sin liable for punishment in the Hellfire, big or small, unless repented for. I'm not sure if this fits in with your technical definition of "mortal sin" or not. Repentance involves four things - asking Allah for forgiveness, feeling remorse for committing that sin, resolving not to repeat it, and if it involves the rights of others, then redressing it (e.g. returning stolen property, money, etc). It is our love for Allah and for pleasing Allah, and our fear of displeasing Him that stops a Muslim from sinning, if a person has 'taqwa' (loosely translated as being God-fearing).
Scholars have mentioned the "kaba'ir" or major sins (lying, stealing, killing, etc) however my teachers state that even committing a small sin and deeming it insignificant can be as bad or worse than committing a big sin and having a true repentance over it. Ultimately we don't know what sins Allah will take us to task for, so we resolve not to commit any sins but do the best we can, and intend to fulfil all our religious obligations, and do what we are able to.
Some Catholic apologists have noted that Muslims believe that Jesus Christ will judge the world and therefore also Mohammed. However, in a quick search of the Qur'an I've found nothing to support this claim. Is this belief found in a hadith? If yes, is this hadith widely accepted?
I have never heard that Jesus will judge Muhammad (although I don't admit to knowing every hadith). From what I understand Jesus on his return will follow the law as brought by Muhammad, as that is considered the final revelation for all mankind, and there is no new prophet after him. There are a number of hadiths relating to the end of time and from what I understand they are somewhat hard to interpret and of various degrees of authenticity.
Is there a definitive list of hadith all sects of Islam accept?
Not all sects no, most sunni scholars accept the same hadiths and their gradings, although there are sometimes differences on how they interpret them. There are many examples - one that comes to mind is when the Prophet pbuh came home in the morning and there was nothing to eat, so he proclaimed that "he is fasting". Now this could be taken two ways, linguistically it simply means that they have nothing to eat, and the other possible meaning is that he intended retrospectively to perform a religious fast (i.e. refrain from food from dawn to sunset). Based on this ambiguity in meaning (but not the actual wording) some scholars have said its permissible to retrospectively intend a religious fast (provided that one hasn't done anything already to break the fast) while others have said that the intention to fast must still be made before its commencement by taking the linguistic interpretation.
how do you morally justify pedophilia?
Firstly I should point out that this is not an article of faith for Muslims and it's not something that they are going to be questioned about. Some Muslims have said 9 and some have said 16 based on other hadiths, either way it's inconsequential as Islam prohibits the consummation of a marriage before a girl has reached puberty, as a bare minimum (and along with other conditions). Furthermore the other of his wives were all older and widowed and therefore it can't be argued that the Prophet had a primary or exclusive sexual interest with young girls, so pedophilia is not a term that is applicable in this case. Muslims don't consider it as such, at the very least.
Ultimately there is no evidence anywhere that the marriage between the Prophet and A'isha was anything except completely acceptable and in line with social norms of the time, nor does it stop a community or society today from setting a higher age limit if they deem it necessary, as is the case with most of the Muslim countries. As I mentioned, there are other conditions to marriage besides age and based on these it is certainly not a recommended practice for Muslims of this day and age. Having said that, if such a community and time existed somewhere in the world where the conditions were met then it would not be prohibited in the sense that a person would become liable for punishment from Allah. This is often the case in Islam, where Allah sets the absolute limits which covers all possible situations but gives us freedom to move within those limits to whatever benefits society as a whole.
Hope this helps,