I've not seen anything that tells us when his leave was due to end - just that he was on pre planned leave on the 4th.
I agree seems weird to have an email re: stress on the 5th, then a return to work so soon.
I also think emailing saying you don't want to work with firearms on the 6th seems an odd thing to do given thats the unit you are on, as it sort of draws attention to you when you'd surely not want to have attention drawn to you in this case. Unless - the email was a follow-on from the stress email on the 5th? Normally being off work for stress means you'll be off for a longer period, and there's a possibility the email about not wanting to carry a firearm being a 'actually I don't think I need lots of time off, but perhaps less stressful work rather than carrying a firearm/patrolling'.
I agree being back to work on the 9th seems odd if he was 'signed off' with stress.