I have dug a bit further for you and found this info from a German site (Google translate - square brackets my notes). It seems that the poor test result may be due to the reclined position of the seat, but also that with a larger child, the whole seat moves quite a lot in impact which can cause a problem. Note also that this testing was done on the original version of the Come & Go, not the i-size version - it may be that the i-size version is improved, although generally it only means added side impact protection. However, if a more upright position is allowed for rear facing in the newer version, it is likely that using that position will give better safety. I would also from this make sure your front seat is as far forwards as it can go.
"These are the backward notes of the MultiFix Twist:
For this reason, we asked the ADAC for the individual grade for the rearward- facing installation of the MultiFix Twist and are disappointed: Unfortunately, the seat only achieved a 3.2 in the frontal impact test even when installed against the direction of travel . The MultiFix Twist was only tested in the lying [most reclined] position, because according to the instructions, only this may be used for rearward-facing installation.
The load on the neck of the Q3 dummy (roughly equivalent to a 3 year old child) was 1,942 N in the test . This is a significantly higher load than we know from good reboard [extended rear facing] child seats. The seat shell yields a lot in the event of a crash, which means that the child's head moves far towards the front seat and there is an increased risk of an impact on the front seat. It is therefore also questionable whether the MultiFix Twist in an upright position - which according to the current instructions is not permitted anyway - can achieve significantly better values.
The seat was not tested with the Q1.5 dummy [18 month old], as experience has shown that backward-facing seats achieve the worst values for the largest possible dummy that is approved for the seat. Any good values for the small dummy, however, do not comfort the only satisfactory marks for Q3, after all, the seat should be able to safely protect a child in all variants and until the end of the period of use.
Heyner's MultiFix Twist passed the ADAC test with an overall grade of 3.2 , but it does not meet our requirements."
www.familiebaer.com/kindersitztest-2019-adac-stiftung-warentest-ergebnisse/
Cosatto don't trade in Germany, hence why they are not listed at the bottom of the review, but Welldon is, and you can find other references online to Cosatto = Welldon. (You can also see it's the same seat with different fabric, if you compare directly pictures of the Cosatto and the Heyner Multifix Twist).
It's a bit tricky because Mothercare can't be seen to favour any one brand over another. And it's not always easy to find out which brands make and test their own seats and which are rebranders.
Rear facing is still safer than forward facing in any seat, so if you can't afford another seat at this moment I would keep using it and keep rear facing, in the most upright position that the seat's manual allows and your DS can tolerate (if he is still little, recline doesn't matter as much, and it's better for him not to be chin to chest) and with the front seat as far forward as it will go. The load values they quote for the seat are still good in comparison to a forward facing seat, they are just poor in comparison to other rear facing seats. You can put neck loading values into context with the graphic in this article:
erfmission.com/crash-course-crash-testing/#:~:text=Swedish%20PLUS%20Test&text=This%20is%20a%20voluntary%20test,neck%20in%20a%20frontal%20collision.&text=The%20crash%20forces%20the%20neck,facing%20%E2%80%93%20compared%20to%20rear%20facing.
People are recommending the Joie Stages, but I can't in all conscience tell you this will be better! The same website has a breakdown of the results for Joie Verso, which is the rear facing only version of the Every Stage, and they are fairly similar - slightly better results on the neck loading but not by much. Joie seats are wonderful - rear facing affordable at any budget - but it doesn't make much sense to replace one "not the best rear facing seat" with another. It would be a good replacement if you were using a cheap forward facing seat.
The cheapest plus tested seat is the Britax Max Way.