Err, the Persian empire was the near east, so to speak, and China is the far east. We're talking about a great difference in distance here.
Besides, he practically took over the Persian empire and its political machinery. That's not going to be the case further on. And you might want to see how the Greek Baktrians ended up if you think dealing with the steppe tribes was going to be that easy.
Also, Alexander was not invincible. He had trouble dealing with a minor king like Porus, and with help from Taxila as well. One of the things it shows is fighting very foreign people with a different enough sort of warfare on their home territory wasn't a simple exercise even for a genius god-emperor.
Alright, I won't waste our time trying to argue subjective things. Each of us will assign a different weighting to that depending on what makes up our point of view, and it is a lot of effort to change that. Usually it is just a matter of emphasis which can easily be misunderstood. But when I see an opportunity to clarify I will point out:
1. Alexander has already assembled his army in Bactria after completing the consolidation of his empire. It did not require marching half-way across a hostile continent the way it did the first time - he's assembling and securing his frontiers as he picks up more supplies and some of the better troops from his 'satraps' along the way. The next leg is actually shorter than the first time he came this way, not longer. So what's this great insurmountable distance ?
2. The Greek Bactrians subsequently conquered about 50% of India, and after rival Successor generals stabbed them in the back, the Sakae invaded Bactria while they were busy elsewhere. In the grand scheme this short lived dynasty did more than most people are even aware of or could believe was possible with such powerful enemies.
3. He had trouble because he had to win a hard fight (with smaller numbers actually engaged btw). Yes it wasn't handed to him on a platter. Having to win a great victory over a powerful king, after they just trounced three equally powerful native kingdoms should count as reason to believe he can do it again.
Anyway, I said very clearly that I dislike over-emphasizing the repeating crossbows and now you're telling me I was extolling their virtuesI dunno, but this thread has been consistently disappointing.
Yes - after a few discussions we both agreed the smaller cho ko nu was not the weapon of choice for the trained crossbowmen (although you may be underestimating it as a weapon of mass numbers, I did not assign a huge weakness to it based on the video - you did !). But it is indeed very dissapointing because this is what you just posted: "They also assumed the Chinese had mainly small Zhu Ge crossbows with pencil-like quarrels that relied on poisoned tips to kill unarmoured targets partly because of a TV show of dubious scholarship."

Without reinforcements from Macedonia, he would have too few elite infantry to execute the Macedonian tactics properly. He would hardly stand a chance, unless he pulled off something really brilliant while his Chinese opponents must be completely idiotic. And I like how he "treats with Tokharians" just like that. And steppe warfare is a whole different ballgame. He would have a nightmare with his supply lines.
No you're not. The point is people are just saying whatever they like here, with some actual info interspersed. But is there any surprise when we're talking about a completely hypothetical scenario while not being established scholars of the relevant fields? We even have people who get their info mainly from the mass media!
After I spend hours building a plausible case, it's so easy for someone to just take my mere reference of what I spelled out elsewhere, as 'just like that', an oversimplified assumption. Appearing as a saviour to the basin people after overawing or subduing the Sakae, and then recognizing their value and being willing to trade or make promises, offers a good chance of success, especially at the head of the most powerful and unusual army they will have ever seen. If it doesn't work out - then he would have to resort to strongarm tactics, but don't try to tell me that every single nation on the road to China is unable to trade, treat, or ally themselves with Alexander. We've already seen it happen for god's sake in history.
Maybe I am missing your point, but let me vent here for just a moment. Unfortunately, it seems I have to prove in convincing detail every possible reasonable benefit Alexander might have in his favour, to the point where it's ridiculous. And then repeat it every time someone posts like this. But are we supposed to accept at face value that the defenders have superior tactics, metallurgy, greater resolve, longer range weapons, are able to support a million trained men, and have generals more gifted than Alexander because they read Art of War ? Clearly, because I have put forward a hypothetical scenario it must pass the acid test and microscopic scrutiny multiple times, but then that same level of scrutiny applies the other way.
Let's just put aside some of the rhetoric 'both sides' have used, please. I've fired off a few barbs myself. In my last attempt; I took almost every solid suggestion from cypher_101 and yourself, even when it was not supported by what little I've read, because I will admit I'm not an authority on Warring States. There is now a Qin army with almost as many cavalry as chariots, almost as many bowmen as crossbowmn, and not cheap ineffective crossbows either, that in total is twice the size plus as Alexander's army. I adopted this against my judgement in some cases because I would rather err on the conservative side than continue to argue about weapons and proportions. I thought it would be more acceptable to everyone this way, but until you've seen what the array will finally look like, you're not convinced. Fine, but please suspend your judgement until you've seen that.
I also supplied a reasonable, simplified logic flowchart of possible outcomes that does not stretch the imagination, and the big question mark is still what happens in the initial encounters. If we express the thought that Alexander has a chance, it is far too early to call it a verdict of victory. It is unprovable in a hypothetical sense like you said, but is it a reasonable simulation attempt ? Are you still unhappy with the tremendous power of the Qin army I've represented here ? What do they need, short of an 85% chance of victory in the first round ?