Hi Kei and welcome to the forum,
We have an InfoPath Master class that we conduct which includes a module on best practices.
To answer your specific requirements, you'll want to use a workflow with an Impersonation Step to remove permissions on submitted forms and add back permissions for submitter, approval chain and admin staff.
Add workflow fields to your form that include a repeating history table with columns for user, date-time, comment, and status.
Add fields for current status and job level. You can use these to control the serial workflow from within the form to hide-show sections based on who the user is and what the status is.
I do not recommend multiple views for the approval workflow. Multiple views are good for hiding information, but you can do that in a separate admin section that is hidden by default too.
Content types are good if you want to reuse the form in multiple form libraries. For workflow on a single form, your consultant's custom coded solution sounds like overkill. I recommend using forms-based workflow for most of the logic since it's declarative and in the form. You'll use SharePoint workflow to move the forms, email notifications, or change permissions - that's it though.
You can do the multiple levels in the form. We'd be happy to show you this in exchange for a support pack. We also have a webinar we did a while back with a video on YouTube that shows the technique.
The training class may be another good idea since it will cover everything here and much more.
Best regards,
P