Much of which is based on data, statements, projections, legalese, SLAs and pricing provided by the bidder.

How, pray tell, would the purchaser be able to proceed without trusting the information they are given? There are of course (limited) checks and balances to confirm the substance of the bid, but ultimately the bidder is the sole source of the majority of the information.

I've been on both ends of enough tenders to know how it works.

The first part of your post is correct.

However, regarding "the purchaser to satisfy themselves of the integrity of the bid.", I dont believe this is always the caes and there are circumstances where, if the information given in any tender is proven to be incorrect, there will be a get out clause for any supplier or, as usually happens, they carry on until the end of the tender period and use the experience to guide them in future deals with whichever company.has "wronged" them.
There can also be occasions when "fixed" pricing can be altered during the period of supply.

I start from the basis of not trusting a word a bidder tells me. It is up to me to satisfy myself that key information is true. Perhaps my industry is even more dishonest than normal. That's the reason why lawyers make a fortune out of contracts & why procurement legislation is in place.

Given the track record (geddit) of the previous private operators on the East Coast Mainline, I'd have thought that the DofT would have gone through the bid with a fine toothed comb. However, the fact that they wanted to get through before possibly losing an election may have affected their judgement.
After all the previous public operator was producing a profit for the taxpayer. Bigotry and dogmatism were behind the awarding of this franchise. So no wonder they cocked up so badly.

Contractually & legally you are correct. However my job is to identify upfront the lies & dishonesty in a bid & subsequent contract. If I just sat here & said "Don't worry boss, it might be 6 months late & £1m over budget, we've got a thousand students with nowhere to live & nowhere to study, but I've just sussed they lied in their bid so we have grounds for breach of contract" I wouldn't last long. This is the industry & business that gave you Grenfell.

That's all very nice and you are quite right to be as thorough as possible, but ultimately at some point you have to make a decision based on data and information provided by your bidder - some of which you will have no way of independently verifying.

Unless, of course, in your line of work EVERYTHING is on public record and can be disclosed without fear of confidentiality, compliance, antitrust and data protection liabilities. Which would be very unlikely.

Under EU & UK public procurement legislation every single word of communication between bidders has to be retained. A rejection of a bid or objection to an award can be raised by any bidder which will result in full scrutiny of the entire process. Similarly, external funding bodies have audit processes in place to ensure the full procurement process was open & transparent. I am not allowed to have off the record conversations with bidders, it all has to be communicated via a procurement portal & all bidders be informed of answers to any questions raised by other bidders.

And in the real world

In the real world - I am subject to exactly the same rules tigertot has described; that's how procurement with the NHS and LA's works, and despite my best efforts, there is no way around it.

And which world would that be? The one where tenders are accepted with a nod and a wink, an extra brown envelope, and the furious deleting of email exchanges? Tigertot's description of the tender process is exactly how it works where I work, and as bren2k has pointed out, there is no way around it. We actually employ a person dedicated to ensuring everything is done to the letter.