from which university did you get your MBA degree?? for my case, i did my MBA from Iqra University and WES dosnt recognize it. I contacted CES and as per them, they assess degrees from recognized institution that work under HEC. since Iqra uni is working under HEC, i think they will recognize it. However, CES dosnt assess degrees from Preston university as clearly stated on their website.
as far as your question for surety is, one can not be assured what assessment any of the institution will hand you out. WES has been handing over 01 year Diploma degree to candidates from Pakistan who had done BBA hons of 4 years and masters of 1 year. CES on the other hand has given them Master equivalency.
for your 2nd query, I asked them on email whether they only require MBA documents or they need previous documents as well. to which they responded that they only need highest degree which one needs to have assessed. but since i already had my BBA degree and transcripts, along with MBA degree and transcript attested from HEC and were in a sealed envelope. I just dispatched the envelope as it is to them. please note that CES charges 210CADs per credential assessment so if you opt for more degrees to be assessed, the charges gonna go up. i paid 312CADs in total ( for assessment+ delivery to Pakistan+ Tax) for assessment of my MBA degree alone. had i opted for BBA degree assessment as well, cost was about 500+CADs.