1. Why? Couldn't God simply make evolution impossible through a natural and explainable barrier?
Yes, but who in the Bible was fooled by evolution?
It's not until the end times that evolution becomes a viable threat to creationism; and in my opinion, it is going to become even more formidable during the Tribulation period, where the Antichrist is going to demonstrate abiogenesis.
In short, evolution is a doctrine that is reserved for Jesus Christ to put down.
2. Further, why would God want to prevent evolution in the first place?
Occam's Razor.
Evolution takes too much time, and the universe has only been in existence for just over 6000 years.
In addition, why daisy-chain life, when life can be created instantly?
3. If God intervenes to prevent speciation, wouldn't that entail that, given no natural explanation, this interference is an observable act of God – a miracle?
Your key words are: given no natural explanation.
Satan will make sure that those who don't want to believe in God will have a natural explanation for every jot and tittle in the Bible.
Jesus walked on water? simple ... it was a mirage
Jesus resurrected: simple ... it was someone else masquerading as Jesus
Jesus born of a virgin? simple ... virgin means "young woman"
Etc and so ad nauseum on.
4. Does God not care about plants or fruit flies or several other animals that have been observed changing into viable groups incapable of with their original group – speciation?
I don't understand this question.
And isn't this just an admission that speciation is possible if God allowed it to occur?
If that was God's plan for populating the earth with plants and animals, yes.
His method, however, is a form of modified punctuated equilibrium (for lack of a better term), where miracle animals -- called "kinds" -- gave birth to different genera.
In my opinion.