Your tup should be fine with a wether in with him just make sure that they either both have horns or both don't. If they don't know each other already then make up a very small pen that you put them in for a couple of hours. They shouldn't be able to move around at all. This way they will get each others smell on them and won't be able to injure each other. When you let them out make sure the ewes are not nearby so that the tup doesn't feel the need to compete with the wether for them.
With your ewes they can vary enormously as to how long in advance they bag up. First timers may be just a few days, some older ewes or very milky first timers it can be a month in advance.
The main indication for dropping the lambs is their behaviour. Look for pawing the ground, restlessness, some will go off their food. If you can get close enough to see the vulva will be puffy and darker coloured and you may see a discharge.
It is possible to get your ewes scanned with an ultrasound scanner which would tell you if they are pregnant and, depending on the stage that they are at, may be able to tell you roughly how far along they are. In the UK there are specialist scanners who travel around or vets sometimes do it.