I'd like to focus my answer on the implications of the recorded static and its meaning for the future of the world being presented in the film.
Lets start with what is known, and some easy assumptions
- Known: The recorder is actually not destroyed.
- Known: The recorder effectively records 18 hours of static
- Assumed: The alien civilization / community of civilizations who meet with potential incomers into the galactic community would have had experiences beyond counting of orienting new civilizations to the wider community of the galaxy / universe.
- Known: Dr Ellie Arroway is extremely competent as a scientist.
- Assumed: Dr Ellie Arroway is emotionally, stable, and trusted by the Earth community as an emissary for all humanity. Even if there are those who doubt her, she will be believed by at least billions of people.
- Assumed: In timescales that measure beyond human lifetimes it is only a matter of time until the news that the recording included 18 hours of static will be released to humanity and that is powerful evidence beyond the point of reasonable denial that will drive human advancement for hundreds of years.
- Known: The alien entities have demonstrated ability to "read minds" by creating a virutal experience of comfort for our protagonist traveller. The aliens meet Dr Ellie Arroway as embodied versions of her own father. The technology is such that these entities are able to distinguish language, read and understand the complexity of language and human relationships from a single brain sample.
- Assumed: The alien entities will have a full understanding of the sum total of Dr Ellie Arroway's memories and intelligence. They will know about Galileo, World War 2, Hope, Love, and everything else in-between.
So... with these assumptions...
The alien entities will know that leaving 18 hours of static will inevitably be scientifically observed. Eventually, perhaps in a generational shift as human-mindsets grow and shift humanity will become more ready to join the universe-community.
The alien entities, we must conclude have had millions, or billions or possibly even quatrillions of attempts to join the universe-community. We know this because the aliens existence shows that life is, at least relatively common in the universe. Given there are 100 billion stars in the Milky Way, and between 100 billion up to as many as 2 trillion Galaxies. The number of opportunities for intelligence is clearly immeasurably large.
Comparatively, human experience of trying to join the universe-community, as pictured in Contact is currently n=1. This is our first try, and we have no idea what is needed / the process to join the universe-community. The aliens will know virtually infinitely more than we for what is the best pathway for connecting with the universe-community.
So to return to the question, do the aliens take steps to assure no modifications to the machine.
The aliens have had quatrillions of experiences, with a 100% mind read on each successful traveller and the understanding of why, how, and if any modifications have been made.
The aliens will even know about modifications that caused failure of the device, and had eons pass since any change has been made. The aliens would discover this information, logically, in this way. In these quatrillions of experiences the aliens have interacting with travellers. Some of those travellers will be "second time travellers" where one device was built with modifications that caused failure, and then that civilization built a second attempt without modifications, and they were able to travel. These second time travellers would have significant knowledge of the first failed attempt. That knowledge would then be telepathically known to the current operating entities that interacted with Dr. Arroway.
In the time scales / and scopes of galactic time, even the alien entities claim ignorance as to the identity of the architects of the universe's travel infrastructure. (This is quoted in the film - said by the aliens). As such we must assume that the design and intake process is timeless in ways beyond imagination.
The structure of the construction, and person-pod as well as its blueprints. Every factor that comes up, will have been scrutinized at time scales trivializing our distance from the dinosaurs.
Such simple modifications as, recording technologies, or a chair and harness are so mundane to the universe-community that built the designs Earth receives. Such simple modifications will have been seen and observed trillions or more times. One can imagine the alien whose day job it is to receive todays new batch of "first contact arrivals" saying did you get 5 chair adders today? Yep same today as the last, had two more fish tanks as well from some intelligent cephalopods.
As to the second question, and to answer with a simple yes/no. Humanity receiving the information / evidence of alien existence that is 18 hours of static means that humanity is ready. Also the alien entity actually talked to Dr. Arroway. That is two pieces of evidence we are ready now. What ready looks like is the next question.
In the Left Hand of Darkness, Ursula Le Guin shows Genly Ai, and the concept of first contact. What is the best way to make first contact. Providing two pieces of information (the ones I just shared), her experience, and the 18 hours of static - shows that yes we are ready to begin the next step, whatever and more importantly, however long that step takes.