OP you are correct about the exception for three children in the middle.
However you are (slightly) wrong about the min age/height for backless booster seats. The 125cm / 22kg only applies to new model approvals. The design of backless boosters really hasn't changed very much if at all since the 1980s, so this isn't a category where new models are being approved all the time (like for example infant carrier seats).
There are dozens of models already on sale which were approved before 2016 and so are legal to use from 15kg upwards. Generally I would not advise this and would say you should stick with the 125cm/22kg anyway, but this is a good example of an exception to that rule, because backless booster is better than no booster.
There is also a sort of unwritten rule that says if the law says you can legally do a less-protective solution, then there's nothing stopping you from using the more protective solution if everything fits.
Lastly I doubt police officers are carrying around a tape measure. If you get stopped they will see the outer two children in child seats, the middle child in a seatbelt, tick a box and wave you off. It would be different if it was a toddler bouncing around in the middle with no seat - they would likely stop you then. But nobody is going to quibble over 1-2cm.
But I do agree with others that if we can help you find a solution with three proper seats maybe that would be better? Someone recently told me on another thread they managed to fit Kinderkraft Junior in between two other child seats. It's not a seat I'd normally look at, but for the middle, the most important thing is the belt positioning, not the side impact protection.
Maxi Cosi seats are also very slim. You may actually find that a backless booster doesn't help because the problem is the bases of the other two seats, not the shoulder part.
There are some infant carrier car seats which have belt fitted bases. This was the key that I found when we had to fit Group 1 seat + infant carrier + teenager/adult in the back of a similar sized car.
Is your 4.5yo in a high back or harness?
Multimac is not as disastrous as a lot of people say - I don't know why it's suddenly attracted a load of ire. That might well be a good solution in your situation as your older two children are over 4 (so would be fine forward facing) and the baby can rear face up to 13kg in the baby seat which is usually over 2 - I did have a photo of a small 3 year old in it. It is probably a lot of money if you were to then stop using it after 2 years though. There are definitely some downsides, the low rear facing time, the baby seat is not really portable and won't go on a pushchair, the belt fit for children who have outgrown the harness isn't fantastic. It's 15+ years old so it's never going to give you the protection of 3x cutting edge modern seats. But it did use the best safety features available at the time and therefore is better than 3x basic/cheap separate seats that you can buy today, which are probably using the safety features of 20+ years ago.