I'm a EU citizen who's been living in the UK since the early 1990s and I have now spent more than half my life here. I am considering applying for full citizenship. I have a blue UK permanent residence document. At the time I applied for it I didn't declare a long-forgotten caution for gross indicency from my first few years in the UK, as I had wrongly assumed it would be spent and I would not be required to declare it. My application wasn't queried and I received the document. I have since gained a better understanding and requested my PNC record, which I have now received. I have several questions.
1. Will my PNC record have been checked at the time I applied for permanent residency? Is there a way I can find out from ACRO if it has?
2. If it has, will it be checked again if I apply for full citizenship?
3. The offence is GROSS INDECENCY (BY MALE WITH MALE OTHER THAN OFFENCE CLASSIFICATION SX56021) without further details, which are held by the arresting police force. Will the citizenship adjudicator seek the full record of the offence from the force?
4. The offence cannot be disregarded as it's recorded as taking place in a public lavatory, which is still a criminal offence under the Sex Offences Act, 2003. This detail doesn't appear in the PNC record but it will in the full local police file. If this is checked, how will it impact on my application? And will I then be retrospectively placed in the Sex Offenders Register?
5. There has been guidance from the Association of Chief Police Officers on the policing of public sex environments issued in the late 2000s. This advises an approach commensurate with public distress or alarm. The arrest took place in the very early hours in a deserted area. The arresting officers were lying in wait, so it had elements of entrapment. They arrested me and the other person, took us to the police station and put us in separate cells. I never saw the other person again. I was carrying an unlabelled bottle of poppers which they then used as justification to gain a judicial injunction to carry out a search of my residential premises, where I was a lodger sharing with my landlord, who at the time was fortunately on holiday. I felt I was placed under duress and was locked up until the early hours. As 6 am approached, and I presume the end of the officers' shift, I was offered to be cautioned. I was advised that the other suspect had admitted the charge earlier in the night, been cautioned and released, and it was the quickest way for me to be released too. I just wanted to bring the whole episode to an end and accepted the caution. I have since realised the long-standing consequences of that course of action. At the time I was cautioned I didn't receive any legal advice. I had been visited by a sleepy, uninterested duty lawyer earlier in the night but a caution wasn't discussed at that point. If I now was to request for the caution to be repealed, what chances do I stand? Of course I would only make the request if the caution was likely to fail my citizenship application. 
6. I would very much prefer to keep this whole episode in the distant past where it belongs and carry on with the rest of my law-abiding life. I'm now anxious however it may constitute grounds for deportation. Is that a possibility? 

Depending on the repercussions of all of the above, I may conclude the best course of action would be not to pursue a citizenship application and save myself the big expense into the thousands of pounds that it constitutes. 

Thanking you in advance for any constructive and informative replies. 

PS - As for the act in the lavatory, I honestly can't remember what was recorded in the caution. I can't even remember if there was any body contact as my memories of the moment, and it was a moment of seconds, were superseded by the events of the rest of the night. And it definitely wasn't conduct than, unless being looked out for, as the police officers were, would have constituted cause for distress or alarm. 

PS 2 - During the home search, the officers also retrieved details of my employer at the time, which also show up on the PNC. They were irrelevant to the charge but I felt they were included as an intimidating tactics.

It's best to read through this: https://www.gov.uk/settled-status-eu-citizens-families/what-youll-need-to-apply

"You do not need to declare any of the following:

• convictions that do not need to be disclosed (‘spent convictions’)
• warnings (‘cautions’)
• alternatives to prosecution, for example speeding fines"

This is different. P is applying for citizenship. What you posted is relevant to Settled Status.

I have recently been through the process of gaining citizenship. You will have to declare everything. Peruse immigrationsboard.com, plenty of useful information on there.

According to AN guidance on good behaviour and criminality:
"You must give details of any cautions (simple or conditional), warnings or reprimands

I don't want to sound unpatriotic, but would citizenship be worth all that expense? If you can get Settled Status, then are the extras worth the outlay? This site has an interesting overview of the question, but unless you want UK citizenship for children, or you really want a British passport, why bother?

=========================================================================================================

If you are to punish a man retributively you must injure him. If you are to reform him you must improve him. And men are not improved by injuries. (George Bernard Shaw)