2) You misunderstand. The commerce clause prevents one state imposing economic sanction against, or prohibiting the flow of goods, people, etc from one state to another. That is its purpose, and was created expressly to prevent the painful fracturing of the nation through economic strong-arming. Thus, however more dense a specific state is populated compared to another state, has no bearing on the laws of one state being imposed upon another. With a centralized power structure, that goes away. It is attempted to be mitigated through re balancing the number of electors (and representatives), but that does not resolve the problem of more populated states overwhelming less populated states on the legal level via the central government. EG, "Both the east and west coast states all are for-- you heartland yokels just need to STFU." That would not happen with state primacy. The power of those coastal states ends at their borders in such matters.
3) Given sufficient transmigration, it would not matter how they gerrymandered inside their state. Eventually, the vast majority of dissenting constituents would have migrated, leaving only the dissenters that were raised there and have yet to move. Given sufficient levels of flagrant corruption, there might even develop migration aid support organizations to facilitate relocation. The act of relocation itself can be a very powerful form of political protest. (See the attempted succession of the southern states from the US, and the resulting civil war, and tensions that have persisted to this very day.)