Hi Marion I have just returned from this trip so thought I would give you some info in response to the things you have raised. There were no signs of any trouble in Cambodia or Phnom Penh & we walked around PP without any problems.
We had no problems with mossies. I was in shorts all day & just used Deet spray & as far as I'm aware no one in the group got bitten by them.
The weather is hot & humid & we had rain sometimes very heavy late afternoon/early evening so take a small umbrella with you although as you are going later than us you may get better weather.
On some of the trips to Palaces & temples shoulders & knees need to be covered & some of the women took a light ling sleeve top to slip on. For men it was more relaxed & shorts at the knees were fine.
The holiday was superb & I'm sure you'll have a great time. You'll especially enjoy the drive from Siam Reap to Phnom Penh if it is anything like we had. I won't spoil the surprise for you !!!
Have a great time. Keith.