The ethics of decision making under time pressures… what would you do?

I would speak to my manager and explain that with the timeframe there was no room for considering consequences and ask for an extension. I think if we value the customers and want to provide excellence, the speed of delivery should not be prioritised. And the cost of mistakes and the negative impact on our reputation far exceeds the project costs.
I'd also find colleagues who share this thought to join me in the conversation.

I definitely think the situation needs addressing or the risk of missing something/making a lower quality decision could have wider and far reaching ramification - beyond the end of the project.
The loss of the corridor conversations means you have to be much more intentional about seeking others out for connection and input.
I would implement some components of the agile world and include a daily start up and maybe an end of day one as well but with a difference. Intentionally use this time to talk about decisions that have been or need to be made and gaining input from the different project teams. Where outside input is needed then task individuals to actively reach out for that input and bring that to the next meeting. This should be done via a call or equivalent and not by an email that could be ignored. If possible, inviting others to those discussions can save time as well. Would also build in a weekly reflection - doesn't have to be onerous in time but critical to review progress/decisions/outcomes and reflect on any other ways to do things or include other considerations. Also - using a collaboration tool for the project to work from can help to capture everything where all involved parties can see and contribute. This can save alot of time when everyone uses it effectively. I would also be raising with People Leaders/Accountable people around the KPIs and to highlight where the time pressures may negatively be impacting the outcome and gaining agreement on the best way to achieve them. Not an easy conversation but is important to atleast try to have in an open and honest way.

A couple of ideas: First, try to insist on adding some project KPIs around the quality of the solution and customer satisfaction, rather than just delivering on time. Point out how deficient a set of KPIs is without a quality control element and 'the customer's face'. Guaranteed the firm will have something like "our customers come first" in its stated cultural values, and project KPIs need to reflect these values (or reveal them as inauthentic).
Second, paint for the decision-makers a real picture of how the risks re rushing this project would impact them personally if they come to fruition. Nothing will motivate a decision-maker more than understanding what they personally have at stake (promotional opportunities, reputation, trust, money, repair time) if things go wrong.

Without the time to effectively reflect and consider your actions, there is a high risk of failing to solve the root issue. The time pressure may narrow your focus to only solve a symptom of the issue as opposed to the issue itself.

To mitigate this risk, it is essential to be honest with yourself and your stakeholders that to correctly complete the project you need more time. Of course, this is easier said than done but in the long term you may be saving time and money by solving the root cause.

If an extension cannot be granted, then a post implementation review or some form of a retrospective will show the importance of including reflection and consideration in future projects.

Interesting dilemma!