I would greatly appreciate if anyone can help me with these questions
How do we accept the invitation in the express entry profile? Steps of submitting the application online?
There are buttons, instructions in the web interface. Just follow those. I hope you have filled forms online. Its nothing different really Everything is inside your "my cic" profile that you login in to.
If the applicant is single, do we have to enter the information of the household members including parents and siblings? I saw a question like that on application
YES, this is done for every applicant. Even if you are a family, you have to fill it for your parents and siblings. This is important for all sorts of future references / sponsorship etc.
Do the educational degrees need to be attested somewhere? Degree is from Pakistan
You will have to get ECA done. All details of attestation are part of ECA instructions. CIC only cares about ECA which is the eventual outcome. But if the degree is in a different language, you need an attested translation along with ECA.
Are the documents mailed out or can we upload them in the EE profile itself? I cant figure out how to get to the document check list.
You can upload every thing in your EE profile. Are you done with filling all forms online? The checklist is generated after that, based upon your answers and is unique for each person. Example if you list 3 employments, the generated check list will have 3 entries for uploading employment letters A, B and C. You can always save and go back and forth between forms and documents to upload. Mailing part only comes when you get a passport request.
Will copies of everything work or they need to be original?
PCC, Medicals, IELTS, WES eval, passport copy with previous travel, job experience, pay stubs and education degrees are needed. Did I miss anything in this list?
I am not sure you understand this, every document you provide to CIC (unless you receive a passport request), is an ELECTRONIC copy which is a scan of original. Now scan of photocopies might work for some docs, you cannot rely upon that and every one uses scan of originals. No one can tell you if you missed every thing or not. Your application might require docs based upon answers to your questions while filling profile. Though i wonder about some generic docs e.g. where is your photo and proof of funds.
If anything is missing from your application do they ask for more evidence or simple reject it?
Depends upon WHAT is missing. If its a mandatory document example PCC (or you submitted PCCs but they do not cover the ENTIRE period since the age of 18 to date for every where you have lived for 6 months or longer), they would likely refuse the application. If it is something additional which they think they need now and wasn't part of initial required docs / checklist, they would usually ask you for evidence. This is mainly up to the visa officer. But only completed applications (basic required forms and attached docs) reach a visa officer.
Thank you