Hi nat, welcome to the thread 
Sorry to hear you've been having such a rough time. Re the weight loss, easy for me to say but I really wouldn't worry about that - when my anxiety was really bad I completely lost my appetite with my churning guts so also lost weight.
Have you taken it yet today? I would say its fine to take it a few hours earlier in the day - sometimes I forget when I took it the day before but always take it between about 9 - 11 in the morning, sometimes later, and never notice any effects.
Not sure what to say about your doctor saying you should take the whole 50mg straight off. I took 25mg for the first week because i was worried about side effects, but I didn't have diazepam to help me through. I suppose he is just thinking that you're in a bad way and need to meds to start working ASAP - so perhaps by taking the full 50mg with diazepam to help keep you calmer, you won't need to start on a smaller dose?
What about if you try 50mg today with the diazepam & see how you feel? What dose of diazepam do you have? I've only been prescribed it once, and it was a low dose. But when took it i was so, so out of it that nothing would have worried me
.
Anyway, let us know how you're getting on...