Were all of the Sahaba capable of committing sins and lying? If they were capable of sinning, could they have committed major sins?
The Sahabah were capable of committing sins including major sins.
There are examples in the ahadith of Sahabah that committed adultery and were punished for it, and adultery is a major sin without difference of opinion. See one example.
But, the agreement of the Sahabah can be considered infallible because it is a form of consensus.
The Sahaba as individuals were not infallible, hence they were able to commit both minor and major sins.
العصمة: بمعنى حفظ الله للمكلف من الذنوب مع استحالة وقوعها منه ... فالعصمة بالمعنى الأول لا تثبت إلا للأنبياء، والملائكة
Infallibility means protection granted by Allah to an accountable person from sins in a manner which makes it impossible for them to commit sins ... and this is not proven for anyone except the prophets and angels
ولم يعصم الله إلا الأنبياء والملائكة
Allah has not made anyone infallible except the prophets and the angels
أن العصمة لا تكون إلا للأنبياء عليهم السلام، وأما من دونهم كأصحاب النبي صلى الله عليه وسلم فليسوا بمعصومين
Infallibility is not (for any human) except the prophets may peace be upon them, as for others such as the Sahaba - they are not infallible