1, the ex-colleague of Watson, Watson asked him where he was living now (so he didn't know) but later Sherlock deduced that the nurse woman had seen the wedding invitation. How did he have an invitation if they didn't know where he lived?
The army would have a record of their soldiers and retired soldiers - invitation would have been sent through them presumably, or sent to his old address and then mail redirected.
2: Why did the photographer go on dates with the women who worked for the ex-soldier. What did he get from them?
He was seeking revenge and so he wanted to find info and a vulnerable time at which he could strike. One date was enough. If he didn't find anything he wanted - then he moved on.
- Sherlock said that all the women had a secret. What was their secret and how was it relevant?
Their secret was that they all worked covertly for the same man.
- How did the photographer get the job as photographer.
Not difficult to find where the photographer worked and then nobble him. Temporarily put him out of action somehow.
I think there was more but that's all I can think of for now.
All in all, quite a disappointing episode. I liked drunk Sherlock and Watson, but as PPs have mentioned it was wrong for Sherlock to become self-aware regarding his social skills.