Let's say I own a stock for which there is an exchnage offer going on and I decide to accept the offer. In line with the terms of the offer, I will have to deliver the shares in a couple of weeks time. If I sell part of my position after accepting the tender (i.e. after 'locking' the quantity I have to deliver within the offer), does this account as a short sale?
No, as long as you buy shares again in time to deliver them under your exchange agreement. In what you described you never borrowed shares to sell them to someone else.
If you don't or cannot buy shares back in time to exchange, then you'll either be in default of your agreement or you'll have a short position, depending on how you resolve this with your broker.