Given a travel related situation, and given a specific question about the situation, summarize the question into a title

Besides the Turkish Lira, can I use US dollars and/or Euros in Istanbul?

I mean particularly for paying in taxis, groceries and vegetable markets.

If this is possible, is there any additional cost for this? I.e. will I pay more if I pay to a taxi driver in US dollars?

What currencies can I use in Istanbul?

My problem: I recently booked a business trip about three months out, noticing slightly too late that there's a huge event (Formula 1 race) happening in town the previous weekend and all the remotely convenient and/or comfortable hotels are asking crazy rates (US$500+).

The usual rule of thumb of booking flights and hotels is the earlier the better, since prices usually go up but rarely come down. In this case, though, I have a sneaky suspicion that hotels may be demanding too much and there may be a fire sale of unsold inventory right before the event.

So: Do hotel prices generally go down right before big events? Bonus points for answers with data instead of just speculation.

(Note: this question has been edited, as I'm more interested in the general question above than the specifics of my situation.)

Interim answer 1: I've followed simbabque's answer and booked a mildly inconvenient and mildly crappy but fully cancellable and not crazy expensive hotel to cover my ass, and will check & report back on the hotel situation before arrival.

Interim answer 2: Idly poking around, I noticed that a big chain hotel near the office that had previously been 100% full had a room free for all three nights at our normal corporate rate (~US$120). Booked! But I'll still check before arrival and report back for science.

Very overdue final update: I didn't do a very thorough check because work, but a casual browse on Google Hotels did show a bunch of hotels offering rooms at less stratospheric prices (under $300). YMMV.

When should I book my hotel if prices are inflated due to a major event?

We have the report of a Ghanaian-American dual citizen, recently naturalized, presenting their new USA passport at the UK border answering "yes" to the question "have you ever had any problems with immigration anywhere" and getting in some trouble (although no refusal).

We have the CBP answering in essence, the two passports of a dual citizen are completely independent.

Is it the person or is it the nationality? Is the question "have you ever had any problems with immigration anywhere" to be understood on its face value, where "you" is the biological person or is it "you as a citizen of the country your passport you are presenting to us" and they do not give this tirade every time because for the overwhelming majority of passengers the two are the same? Is your new passport a clean slate? Or do you carry the problems resulting from the heightened scrutiny of your other citizenship forever? Is it possible for a border guard to swipe my Canadian passport and be aware of the history of my Hungarian passport? How entwined are my two inner citizens :) ?

Let's restrict the question to those who have at least one citizenship in the Five Eyes countries (Australia, Canada, New Zealand, the United Kingdom and the United States) and one of the Five Eyes borders.
Can the border guard match up the passports of a dual citizen?