My understanding from when I tried putting together a RTW ticket (before deciding it wasn't really worth the money- the one in Y was something like 10% off the total of discounted restricted round-trip fares to wherever I wanted to go) is that you can't really do 3-segment flts between places. They wouldn't let me do CAN (AF) CDG (OK) PRG (OK) KRK, unless PRG was counted as a stop. So I don't think your itin is allowed by the ST RTW rules, though it's worth checking (and DL staff might interpret them in a way different from that the AF people I spoke with did). If I were you I'd substitute TUN for TIP and go overland (assuming you can actually cross the border!).