CSL question - Urgent! Please help

Greetings,

I wish to ask for opinions/solutions to a sticky situation I'm in at the moment...

I've got a BSc Mechanical Engineering (ME), and worked for about 2 years on that basis (NOT as a Mechanical Engineer all those 2 years but I was hired for that background)

Then I quit, for my MSc Petroleum Engineering (PE), and since graduating from that it's been 1.5 years since I'm working as a Petroleum Engineer.

Given that ME is on Critical Skills List, and PE is NOT:
Which occupation is recommended for applying for SkillsAssessment and then subsequent application to DIAC??

Another question is... (and I'm guessing the answer is Yes already, but still would like confirmation)...
Do I have to choose the same occupation for SkillsAssessment and DIAC application?

Please respond as quickly as possible. Thanks in advance and I appreciate your attention.