Since the American president avoided answering this direct question, saying only that he wants “the conflict to end” and at the same time adding that in his humble opinion, Russia is ready for this end, unlike Ukraine, I will have to answer in his place. 

I'll start from afar.

When he was 12, a boy named Menachem Binyamin Zivotofsky was nationally known - because he started suing the United States much earlier than he learned to read and write. Not by his own will, but thanks to his parents, who, right after his birth, made the Zivotofsky surname precedent-setting. The fact is that the boy was born in Jerusalem. Ari and Naomi Zivotofsky wanted to put him on an American passport as an Israeli citizen. Impossible, they replied, although since 1948 the United States has recognized the state of Israel, but the boundaries of this recognition do not include the city of Jerusalem. In order not to aggravate the already eternally hot situation in the Middle East, the U.S. State Department has established the writing of “Jerusalem” as the place of birth for people who were (not?) lucky enough to be born in the location, the struggle for belonging to which has not subsided since the beginning of the state of Israel. But a few months before Menachem's birth, in 2002, the United States Congress passed a law, according to which American citizens born in Jerusalem have the right (by themselves or with the help of their parents, if not yet grown up) to name Israel as their birthplace in their American passports. In signing it, George W. Bush added a special statement (signing statement) in which he noted that this provision was not enforceable because it interfered with the president's constitutional authority to recognize foreign countries. That is why his State Department refused to “Israelize” American citizens born in Jerusalem. Menachem's parents, as they say, were seriously steamed by this legislative limbo, joined by numerous Jewish organizations, which of course sided with Congress. The case was long dragged by different courts and finally ended up in the Supreme Court, because it came to an extremely sensitive issue of the distribution of the branches of American power, which could not be solved unambiguously throughout the history of the United States. So, who, Congress or the president, recognizes the existence of foreign states within certain borders? In 2015, already under Obama, the case of Zivotofsky v. Kerry was finally put to rest. The Supreme Court decideв that the president has the exclusive right to recognize foreign states and governments, and Congress cannot force the executive branch to contradict its policies through legislation. So based on this decision, Donald Trump has the complete and total right, without consulting anyone additional, to call Crimea a Russian peninsula tomorrow? 

It's not that simple. This court ruling does not apply to legislation passed by Congress before the recognition occurred. While bills like H.R. 463 (the Crimean Annexation Non-Recognition Act), the eponymous H.R. 596, and the newest one, this year's H.R. 1600 (literally intended to limit the president's freedom in this matter) failed to garner enough votes for anyone to sue Trump over Crimea, there are enough laws passed, much more general, that make the Russian annexation of Crimea illegal. At issue is H.R. 4278 (Support Ukraine Act) of 2014, H.R. 5859 (Support Ukraine's Freedom Act) of 2014, H.R. 3364 (Countering America's Adversaries Through Sanctions Act) of 2017, whose 253rd paragraph explicitly says that the U.S. does not recognize territorial changes resulting from the use of force, including the illegal invasion and occupation of Crimea, Eastern Ukraine, and other regions. This, again, is consistent with the Stimson Doctrine, according to which annexation in violation of international law cannot be recognized, which is the basis for the Welles Declaration of July 1940, which did not recognize the annexation and incorporation of the three Baltic states by the Soviet Union, which was the official position of the United States until 1991, when the Baltic states regained their independence. 

So, any attempt by Donald Trump to recognize Crimea as Russian will be challenged as an abuse of presidential power, or even preempted by lawmakers under the very system of checks and balances that all Americanists have heard about for years and retold to others who haven't?

There is one caveat. As of now, both houses of Congress belong to the Republicans. Despite numerous statements about broad bipartisan support for Ukraine, the Republican Party has so far demonstrated its willingness to fulfill the president's whims, including during the four-month blockade of US aid to Ukraine back in Biden's time. So, only people who believed in the real possibility of removing Trump from power through impeachment in 2019 can expect that one of the above acts, which explicitly prohibits the president from cutting off a piece of Ukraine on the new American map of the world, will be quickly passed (and then a bipartisan effort will overcome Trump's veto). The U.S. Supreme Court is also pro-Republican (6-3 in favor of conservatives, several high-profile decisions, including the repeal of the amendment banning abortion at the federal level), and the aforementioned Zivotofsky v. Kerry case plays into the president's hands here. Who, as Kyivites heard last night, has no favorites. 

But Russia...