Yes you can. I do it often

Yes, I do it all the time but if they have paid for 2 lots of postage I usually upgrade them free of charge to Express Post as a bonus. 

As long as the items have been bought from the same account, you can upload the same tracking number to each item, regardless of whether they were paid together or separately. 

I often do it. I always provide an appropriate refund on postage too. EG I sell 2 pairs of jeans with Aust. Post small satchel postage on each with shipping cost of $9.50  means I post the two pairs together in one satchel and refund one lot of postage @ $9.50

Yes,  but I suggest refund a part of the postage so you don't get a grumpy to much postage feedback.

I do the refund accross both items (half to each), so if there is any refund, or other problem,  its balanced accross them