If I do Mandatory Reconsideration could they take away my award all together?

You can be awarded any part of PIP, I also encourage the MR. They can of course take your whole award away although it's unlikely and rarely heard of them to do this. You will not need to attend another assessment. Remember, you have one month from the date of decision to request this although once you have phoned to 'start the process' you can phone close to the deadline to request more time if you need to gather evidence etc. 
See the points here:
https://www.citizensadvice.org.uk/Global/Migrated_Documents/adviceguide/pip-9-table-of-activities-descriptors-and-points.pdf

It may also be worth getting expert advice:
https://advicelocal.uk/

Other members will add further I'm sure although I thought I'd get the basics out of the way. Good luck