I was interviewed in July 2024 when I was at about 650TT
Envoy does not want you to apply too early or too late with respect to your total time. Ideal is somewhere around 600TT
Multi is not yet required, but it's definitely better to have before you apply, as the market gets more and more competitive.

Yes, they are hiring FOs from outside the program; however, it's really competitive now.
Rumor has it that they are currently reviewing a few thousand applications.

To apply as a cadet, it is simple. First, you need to be a CFI working for the affiliated school.
The list of schools is included on their application in the airline's app. Once you apply, it takes a few weeks, and you will be contacted with the interview date. They send you to Dallas at their expense and put you in a hotel. On the day of the interview, the process begins with a review of the logbooks, certificates, and medical records. If you forget any documents, you will be asked to leave and will not be allowed to proceed with the interview.

After that, they will let you know if you have been accepted and provide you with a CJO. If yes than after that you will get an email every month to provide your hours (you must fly at least 40 per month to stay in the program) than once you hit 1460 you send them an email saying that you are ready to start the transition and after that in about a month to 6 weeks as along as they are heaving classes you will get a call with a date.

As of this moment, here is the list of affiliated schools from the airline apps.
Bear in mind that even though some schools are affiliated, they may no longer actively promote that affiliation.

ATP
AUBURN UNIVERSITY
BRIDGEWATER STATE
CAE
COAST
COCHISE
Cornerstone
CTI
Florida Institute of Technology
Heritage Flight Academy
Illinois Aviation Academy
L3
LETOURNEAU University
McAir Aviation
Northwestern Michigan
Oklahoma State University
Sky Warrior
Skyborne Aviation
Trust Flight
University of Central Missouri
University of Dubuque
University of Nebraska Omaha
University of Oklahoma
US Aviation
Utah State University
Delaware State University
ERAU Daytona
ERAU Prescott
Liberty University
Ohio State University
Pursue University
South Estern Oklahoma State
Southern Illinois University
University of North Dakota
Western Michigan University