There is nothing wrong with this as the feed in to the garage is fused at 13A already.
2 RCDs in series makes it very hard for you to test the RCD tripping times when you carry out the essential tests before putting your new work in to use.
The additional MCBs won’t make any improvement as the current protection is correct, however they will have no practical negative effect on the installation.
Yes the armour should be earthed at least at one end, preferably the supply end.
No DP isolation is not normally required.