Hi OP - I am unsure what you mean by the isofix arms not locking, but if they aren't secure then I definitely wouldn't use the seat just in case.
Are they sliding the child seat closer to the backrest of the vehicle seat, or do you mean it is more loose sliding further towards the front of the car? I wonder if you mean you can't adjust it to the rear facing recline position, is that right?
You're right to say baby needs to be rear facing as 12 months, too.
Also have you got sorted with the top tether? This is an important piece which mustn't be forgotten as well.
To be honest I have not heard a lot of good things about the Reecle car seats - I'm not sure their quality control is very good, which is a bit alarming on such a safety critical product. Can you contact the place it was bought from and see if they will accept a return, as it may be faulty? This should be possible even if it's outside the usual returns window.
If you need a replacement seat without paying too much, I would recommend Joie 360 Spin/GracoTurn2Me, they are spinning ones but much easier to install. Or Halfords even have a rear facing seat up to 105cm called Impala which is only £60.