Hi OP hope this isn't too late.
I haven't used one but know a little bit about them.
The minimac is suitable up to 13kg, so essentially can be used until the child is that weight although I'm not sure about the height of it - you could only use it until their head reaches the top. I'd make an educated guess of around 80cm. If your child is on the 2nd centile just for weight then it won't make much difference as they will still end up too tall for it, but if they are on lower centiles for height as well, then it could potentially last longer, maybe until around 2 years old.
You can't use any other car seat when it's installed. The side seats can be used by an adult (or child over 12) with the adult seatbelt but this uses a buckle which is part of the Multimac seat, so it's not approved to fit car seats. Also you would have to fit it on top of the Multimac seat so it would be pushed up and out and potentially the seat would be too narrow.
In its normal configuration it fits children up to 12 (this will be 36kg - unsure of height) but once your DD reaches 12 she could sit in one of the end seats using a normal seatbelt. But again this is very narrow for her and might not be comfortable. My 12yo is skinny and sits in a normal 3/4 sized middle seat between two car seats and finds this uncomfortable - it's OK for short drives around town.
Some other points to bear in mind about the Multimac system in general is that it's been around quite a long time (certainly since DS1 age 13 was little) and hasn't changed the design at any point during that time. So while it did take advantage of the best things in car seat design from that time, some of the features are looking a bit dated/lacking in comparison to modern car seats. The main thing that is lacking is the ability to rear face longer, and side impact protection. It has a couple of features which are good - the harnesses have shock absorbing pads, it also has multiple support legs which help reduce forwards rotation, and of course the harnesses have a high weight and height capacity (36kg).
Side impact though - the children particularly on the side seats are placed quite close to the doors, which is concerning. There is a headrest which provides a little side impact protection for children age 1-6 but once they have outgrown this, there is essentially no side impact protection at all. There was a Which? review for it a few years back (you can find the text of it on mumsnet if you google) and they were very concerned about this. The baby seat has very shallow sides and doesn't really protect the baby's head at all. It's also really upright and the inserts are extremely basic compared to the inserts that come with modern baby carriers.
If comparing to very basic car seats that are on the market now then I'd say it's not a bad option, particularly if you don't plan on extended rear facing and you have children on lower centiles, but if you're the kind of person who wants the best possible car seat and researches to find the best thing possible then it's probably not for you. I doubt they will make any changes to it/release any new accessories now either as the legislation it is approved under is going to become obsolete in about 2 years (it would still be legal to use if you've already bought one). I would guess the company is probably looking at how to create a new product conforming to the newer regulations.
If using the 4 seat model you need to inform your insurance company. You may want to enquire first to check if it would increase your premium.
As for options you haven't thought of - if you're one adult with all 4 DC then you only need 5 seats anyway, if you're 2 adults then you can take 2 cars. I think I'd probably think about how often you realistically will need 6 seats in one car. I don't see that you will need 2x 7 seaters in any scenario, as long as you're both insured on one, so it might be worth costing up Multimac vs changing just DH's car.
As DD gets older she may be less likely to come along to things also. It is not often that we have all 3 DC in our car as DS1 likes to stay at home or go out with friends now he is a teenager.