Wow a question that brings back memories of long ago... most of my education in civics and constitutional history had me diving into those questions. This might be a long one, so I hope you don't mind, but it's kind of a complicated answer.
Checks and balances are supposed to prevent any one of the separate branches from becoming too powerful. Currently we largely have dereliction of duty on the part of one of those branches (Congress) and executives (many over the years not signaling out any) who are running hard on grabbing power when it's handed off too freely and issuing fiats or EOs in the face of that dereliction which depending on your affiliation or perspective at any given moment might seem a bit tyrannical or at least at risk to be perceived that way. Give an inch take a mile as the saying goes.
But when SCOTUS makes a decision that seems consequential on society, it seems no checks exist at first. There are numerous cases over the years that have had huge impacts on our society some good and some bad. As time passes some of those decisions are found to be meritless or wrong and require change because they were harmful or do not reflect standards of the present time - others have remained fundamental good decisions that are consistent with the Constitution and Bill of Rights. We are all human and infallible - mistakes get made... but with unchecked power, those mistakes can be big so that's why it's important to elect the right people.
Judges and SCOTUS in particular are critical in performing a function of curbing executive power, just as Congress also has a critical responsibility in doing so because the founders knew that too much unchecked power meant kings and tyrants could be on the horizon. That does not mean these branches have to always say no, to the contrary, they have the right to say yes or even a mixed bag. They do not and should not be giving out blank checks or always agreeing with whatever someone wants just because a party would like them to.
But let's imagine a world where Joe Biden is president, the entire SCOTUS croaks in an accident and he nominates a new cast of characters. Where's the checks and balances? Well, for one, the President gets to nominate in the first place, and as such the judge in general tends to be a reflection of that. Though I tend to think that a judge should simply be a good one with sound practice and not a political animal (that doesn't mean that judge should be chosen to simply give the executive a blank check). The second check comes on the part of the Senate which gives a thumbs up or down on the nomination, it's also not supposed to be a blank check process. Beyond that, there is little else on the part of the executive branch to do with SCOTUS and that's as it was designed. The rest of the checks all fall to Congress - that has the power change the size of the court, decide what cases they may hear, amend laws ruled on by the court in a positive or negative manner, or impeach. Because afterall, they are the persons nominated to represent the voters. Voiding an executive branch action is essential duty to both SCOTUS and Congress because otherwise the executive is unchecked and ceases to be an executive, but a king. Executives are meant to be making decisions but the rest of the branches have the power to check them by - elections, impeachments in the worst case scenario and voting on bills and nominations. The executive can veto bills, but their vetoes can be and should be overruled if necessary. Maybe that makes some people feel as if the executive is powerless (I'd bet executives might grumble about this all the time), but my gosh I'd say most people think the executive has been quite powerful no matter what party is in charge.
In Fed #78, Hamilton talks about how the judiciary is perceived. He surmises it is the least powerful of the branches when it comes to the ability to make war or the power of the purse - but it is an essential branch of important judgement. It checks the executive, but must also rely on the other two branches to enact what it interprets as fair, or change what isn't. The day the executive decides to ignore that check, and a day when Congress stands idly by, is a bad one. He foresaw potential weak points in the idea and the founders mulled how to fix them.
For the final question about nationwide injunctions well there's lots of arguments: some against them might say it allows a lower court to rule on matters that should be SCOTUS' job (these cases do get there eventually anyway)... but I'd say there's a good purpose for them. If the federal government is causing an injury or harm in all states, it wouldn't be practical to have a ruling only apply to people in one state and not in another. While it is becoming a controversial subject take a look at some of these articles. https://hls.harvard.edu/today/do-universal-injunctions-lead-to-national-rule-by-one-judge/
https://harvardlawreview.org/print/vol-137/district-court-reform-nationwide-injunctions/
https://judicature.duke.edu/articles/one-for-all-are-nationwide-injunctions-legal/
And here - opinions that I don't disagree with.
https://reason.com/volokh/2021/03/26/asymmetrical-nationwide-injunctions/
https://reason.com/volokh/2025/03/14/trump-administration-asks-supreme-court-to-lift-universal-injunctions-against-its-bitrthright-citzenship-order/
In short, executives are always going to be crying about having their power curtailed, congress could fix a lot of problems if they'd do their job to check both the executive and the judicial, and nationwide injunctions are not as terrifying as their critics make it seem - they seem to be beloved or hated depending on the way the political wind is blowing.