Even for the richest person on the planet, buying Twitter was always going to be a challenge- a highly complex financial transaction now made even trickier by a defensive “poison pill” move from the platform’s board. Musk’s $43 billion offer lays out the myriad potential pitfalls: possible government approvals, legal as well as regulatory due diligence, negotiations of a final agreement and, of course, how to pay for it all. Then Twitter’s board on Friday showed it won’t go quietly, saying any acquisition of over 15 percent of the firm’s stock without its OK would trigger a plan to flood the m…