Question: 

Can I use “Is Edit Mode” to lock a form during editing?

For example – someone submits the form and the workflow sends it to a group for review – group is Reviewer A, Reviewer B, Reviewer C. If Reviewer A opens the form can it be locked so Reviewers B and C cannot not edit it simultaneously.

Answer: 

Because there is no Check In / Check Out functionality on SharePoint lists, this one would need to get a bit creative.

There are a couple of ways you can go about it, but this is the way I would set it up:

1. Create two additional columns in the list “Check Out” (with option for Checked-In or Checked-Out) and “Checked Out To” (people or group)
2. Create a calculated value control on the form and connect it to the “Checked Out To” control, and set the formula to “Current User”
3. Add a save and continue button (can be renamed check in /check out)
4. Create a Disable rule and apply it to the Save and Continue button and the “Check Out” control. The formula will be targeting If the control is ‘Checked Out’ and the Current User is not equal to the ‘Checked Out To’ item property (this one may take some tweaking if the postback doesn’t refresh the item property in the form designer).
5. They can throw the rest of the controls in a panel and put a rule on there that disables the controls if the status is Checked out and the current user is not equal to the checked out user.

This is going to be the conceptual way to throw it together, from there are a few different ways they can stylize it to make it fit their needs (IE not using a choice control and just doing JavaScript for the button).