MPL Explained:
The frozen ATPL (not an official term) is just a Commercial + Instrument rating with ATPL theory complete. Approximately 250 hours of flying time in real airplanes, and a pilot may add a type rating to their license with minimal simulator time. When a pilot reaches ATPL minimums, they upgrade to an ATPL.
This is what a MPL entails:
First of all, an MPL cadet is selected by an airline befor the commence training. There is a reason for this
From day one, all training is tailored specifically to the airline SOPs.
Training includes approximately 60-85 hours of airplane flying, at which point they transition to approximately 150 hours of simulator time, the majority being level C and D sim.
Everything is done to a specific airline SOPs. When the cadet graduates and is issued a MPL, they are restricted to flying one type at one airline.
For example, if a MPL cadet is selected by Easyjet, upon completion of training, they can only fly the A320 series for Easyjet, they cannot transfer to another company without significant additional training.
Lufthansa was using the MPL program recently, however they abandoned it because they operate several certificates with many different types, and the graduates were pigeonholed into one type at one certificate.
A MPL pilot does not have privileges to fly single engine or multieengine as PIC. They cannot fly single pilot ops. Not even for private pilot privileges unless they undergo additional training and complete a private pilot or commercial pilot checkride (upon meeting the experience requirements, but with only 85 hours of airplane time, they have quite a ways to go to get a commercial).
Some airlines that employ MPL:
Easyjet (program costs 99,000GBP, and 69,000GBP bond is considered repaid to you over the course of your employment on top of your salary, once MPL pilot has flown 1250 hours and spent 2 years with the company, they can be eligible for full time employment)
Flybe (the airline loans cadets "23,000GBP" to the cadets towards their training, which is paid back from their salary over 5 years, and they still have over 56,000GBP in out of pocket training costs... last I heard, the company had a furlough while MPL cadets were in the middle of training so they were caught in a pickle regarding their training)
AirAsia (this is not Asiana)
BA Cityflyer
Dragonair
Qatar Airways
Monarch
Lufthansa formerly used MPL
MPL has been implemented in Austalia, but not sure which companies
In most cases, cadets are required to pay for their own training.
Once the MPL license holder reaches a specific number of hours of total time, they can use their license in a different company. If they reach 1500 hours TT and meet several other requirements, they can upgrade to a full ATPL.
Most of these require a cadet to secure a significant loan or bond, and they are stuck to a company until they reach at least 1500hours and have either been "paid back for training costs" or have "paid the company back" for their training costs. Some of these schemes where you are "paid back" usually go like this: your salary is about 1000GBP per month, much less than the typical FO salary. You are paid back at the rate of 1000GBP a month for your traiing costs. Each month you take home 2000GBP, in which half is "helping you recoup your training expenses".
Bigger issue I see is getting 60 hours pilots into the right hand seat of an airliner with little to no PIC decision making experience/skills.