I actually believe that international tensions will not be a major hurdle. Mainly because international water laws have long been kept on the books and followed mostly by other nations. A seastead just shouldn’t present much of a threat to any nation that mattered, and those that don’t would not want to anger the nation of origin which is likely to be one of the major countries.
That said, if a third world did decide to attack I believe it would be to TAKE OVER not to SINK. This would likely be for profit reasons. If it is making money the nation might want to expand it’s income.
More likely a Pirate like group would take it over to have some base of operations for some bad activity. Sex trade, drug trade, ect ect ect… these last two types of invasions COULD be held off, if you took their possibility into mind while building. Secure areas for youth, mandatory armed status for the citizens, some group funded minor guard force who’s main job is to do some monthly training with the armed citizens.