Djibsomali
Answer to your first point is that Sacad bin Maalik is Sacad bin Abi Waqqaas and not some other little known saxabi so there's no contradiction because it is one man with two different ways of calling him. Like Abu Bakr was called Abu Bakr, al Siddiiq and Ibn Abi Quhafa and his real name was Abdullah bin Uthman. Still all those names are talking about one man. So there's no contradictions concerning that.
I don't see the problem in your second point. So what if it was this particular saxabi who narrated it and not someone else? Are you questioning every single hadiith that why was this hadiith for example narrated by Abu Huraira and not Anas bin Maalik? What difference does it make who narrated it?
And also there could be that Prophet SCWS said something and there were only few people or maybe one person with him and others didn't hear about it. For example when Umar was khalifa it once happened that he wanted to see some saxabi (don't remember who) and when he came to Umar's house he knocked, but Umar couldn't answer straight away because he was busy, so this saxabi knocked three times and after that he left. Umar came out and couldn't find him. Later when Umar saw him he asked why did you leave, and the man answered that Prophet SCWS taught us to knock three times and leave after that so not to bother someone if he's busy. Umar asked him to bring another witness to confirm that Prophet SCWS said this and the man did.
Also it could be that if someone heard Prophet SCWS praising him, he wouldn't go around telling it to everyone because of humbleness. Also in the hadiith of Saciid bin Zaid he only mentioned the other nine and only after being asked he told that he was the tenth.
Anyway even if all those ten would be present how would it matter which one narrated it? By the way there is another narration about the background of this hadiith narrated also by Saciid bin Zaid and it was reported by Ibn Asaakir. Saciid heard Abu Bakr asking Prophet SCWS that he would like to see a man from the people of Paradise and Prophet said that he SCWS would be from them. Abu Bakr said that there's no doubt about it and Prophet SCWS replied that you are in Paradise, Umar is in Paradise etc...
Those ten mentioned in this hadiith are not only ones who were promised Paradise. There are other ahaadiith about several asxaab promised Paradise (Abdullah bin Salam, Bilal, Jacfar, Zaid, father of Jaabir bin Abdullah etc) and there are also separate narrations from some of those included in the list of ten where they were also promised Jannah.
Those 10 were known to be the best of saxaba, and when Umar was dying he named shuura to elect the next khalifa from among themselves and he chose the best people alive that time: Uthman, Ali, Talha, Zubair, Sacad bin Abi Waqqas and AbdulRahman bin Awf. Abu Ubayda had already died during a plague while leading an army in Shaam and Saciid bin Zaid wasn't named perhaps because he was from the same qabiil with Umar.
