to avoid being left unsynced, it needs to ban the opposition so that it doesnt see the oppositions higher blockheight and also doesnt see thier own attempts getting orphaned and doesnt end up trying to grab the highest height just to orphan and remain unsynced.
Again, this is not true. A "higher block count" doesn't matter if these blocks are not valid.
If the chain contains 5 more blocks of 3 MB, and your node considers only blocks smaller than 1MB, these blocks are simply invalid. You don't consider them. You keep the last block of 1 MB as the highest one. The miners keeping with the old protocol will do the same, and build on this block. So you will now accept this next block. And the next 1 MB block. And the next and still the next. You are on the old protocol chain.
On the other hand, a BU node will accept these 5 extra blocks as valid. It will consider the 1MB block next to it as orphaned. and the next one and the next one. Because if BU has more hash power, the chain built on the 3 MB blocks which you consider valid is growing with more PoW. You are on the new butcoin chain. And there's no confusion. A BU node will only accept the BU chain (and orphan the old bitcoin branch). A bitcoin non-BU node will only accept the bitcoin blocks of < 1MB, and consider the other blocks as invalid blocks. Whether they communicate or not.
But you should seriously distinguish between a soft fork and a hard fork.