First of all, the current block height is over 790,000 so you'd need a LOT more than 10,000 blocks to "be far longer than the main chain".
You can start from the Genesis Block, or from the current block. In both cases, changing the rule from the heaviest chain to the longest chain is a hard-fork change. That means, the starting point is not that important, because it is hard-fork anyway. Also, "drop the difficulty to 1" without going through difficulty adjustments is again a hard-fork. When it comes to this rule, you can even see it in action by looking at testnet3: it is much longer, but it also has much lower chainwork.
Secondly, after you've churned out 2016 blocks, in order to create valid blocks, you'd need to immediately increase your difficulty by a factor of 4, then again after the next 2016 blocks, and again after the next 2,016 blocks.
Not really, because you don't have to put the correct timestamp in your blocks. Also note that difficulty increase is not absolute, but relative. So, if you count, how long it took to mine all blocks from 2009 to 2023, and you compare it to the time we should get there (for example by looking at halvings), then you won't reach the current difficulty. That means, your log2_work could be for example around 40, and your chain could be longer than the current one.
Pretty quickly, you'd encounter a situation where your particular equipment can't mine blocks any faster than an average of one block every 10 minutes. You could shut your equipment off for a few weeks to get the difficulty to come back down, but you'd burn a lot of time waiting, and then once you've spent a few days mining another 2016 blocks, the difficulty would shoot right back up again.
There is no need to wait. You can mine blocks with future timestamps here and now, and release them in the future. Then, someone may even join your network, and try to mine new blocks, but if you will have a lot of blocks prepared in advance, then you can always trigger chain reorganization on those clients.