Before we all get carried away, let's break this down part by part. CAUTION, ALL IMHO!
Part 1:
Relinquishing (including renouncing) US citizenship is not dependent on filing with the IRS. When you renounce/relinquish, your record regards US tax is not a factor. It is a separate factor. If you wish to extract yourself from the US cleanly, legally, and without future anxiety, you will also solve your US tax problem. That need NOT be done prior to renouncing/relinquishing, but there are reports it would be much better for you if you did do it first.
Part 2:
If you are allowed to relinquish, you do not pay the $2,350 fee. The US will insist that anyone relinquishing/renouncing US citizenship must have a second citizenship. Technically, this is not required, but you may have trouble without the second citizenship. If you did not have a second citizenship, you would become stateless, even though you may have the right to remain in a foreign country. Not good to be stateless, but there are 1 or 2 examples of those who have done this.
Part 3:
Relinquishing other than by renunciation. There are 7 ways to relinquish. We'll forget two; your country is at war with the US and an act of treachery. Renunciation is a third. That leaves 4 other ways to relinquish.
Naturalization in a Foreign State: You become a British citizen. If, at the time of naturalising, your intent is to lose US citizenship, it can be an act of relinquishment. It's been reported on other sites of those who prepare a written statement indicating when taking the oath at the ceremony, they are doing it with the intention of losing US citizenship. They take it along to the ceremony, and have the person who administered the oath sign, as a witness, the document. Surprising it seems, most of those administering the oath are happy to sign the document. You take that signed document, along with the other required forms, to the US Embassy, and present it as evidence of your relinquishing act. The Embassy may agree, or they may say it is not sufficient evidence. You may be persistent, and insist they forward the paperwork to the Department of State in the US. The officials at the DoS may accept your naturalisation as a relinquishing act, or they may reject it. It may be many months before you hear back. If they reject it, you then may follow up by renouncing and paying the $2,350 fee.
Oath of Allegiance. Taking an official oath of allegiance other than doing it at a citizenship ceremony.
Join Foreign Armed Services. Generally only works if you attain officer ranking. Enlisting as a squaddie won't suffice. (Think of the dual US/Israel person who leaves the US to do mandatory service.) You must initiate the relinquishment. Being an officer is not sufficient to have your US citizenship taken away, unless the country is at war with the US.
Work for a Foreign Government as a Foreign National. Perhaps the most popular way to provide an act of relinquishment. Work for the UK Foreign Office? Become an MP? Again, as the Boris Johnson case proves, it is the individual who must initiate the relinquishment. Boris is still a USC although he spent time in the UK Parliament as an MP.
Part 4:
If you request relinquishment today, based on a relinquishing act of, say, 2 years ago, it is likely you will be deemed as subject to the completion of your US tax obligation, including filing form 8854 and its' attendant requirements.
Part 5:
The messy bit. In the 1970's, many Americans went to Canada, They became Canadian citizens, and at their ceremony it was made clear they would lose their US citizenship. In 1970, the US did not allow dual citizenship. Fast forward to 1986(?). The US Supreme Court rules the US does not have the right to take away someone's US citizenship. Dual citizenship for the US person is tolerated, but not encouraged. Without knowledge of the fact, those Canadians once again became dual US/Canadian citizens, subject to US obligations. Fast forward again to 2011. Thanks to US actions (the increased enforcement of FBAR obligations and FATCA) those individuals discover they are indeed still US citizens with the attendant obligations. 26 Section 877 did not exist in 1970.
In 2013, they go to the US Embassy to claim a CLN. Most are granted the CLN based on their taking Canadian citizenship in 1970(+), but the paperwork for the CLN isn't dated in 1970, it's dated in 2013; but their CLN date of the act of relinquishment is 1970(+). Do they file an 8854? Most don't. Are they a "covered expatiate"? This is when the date of the relinquishing act becomes subject to discussion.
AGAIN; CAUTION, all IMHO!