In order to attain the Beatific Vision, one must be in a State of Grace. The disposition of the soul is the most important thing. However, a Requiem and burial are for the praying of the soul (not its canonisation as has become the norm with eulogies) and for the proper honouring of the body, which shall rise unto glorification on the Day of Judgment. However, the disposing of the body has no affect on the soul, since, most likely, the soul is already where is belongs long before the funeral takes place!
As to ghosts, since the soul immediately goes before God to be judged and then arrives at its proper place, they cannot be “trapped” on earth, nor unable to “cross over”. A ghost may either be the soul of a person in Purgatory who has come back to ask for prayers, or, most commonly, it is actually a demon, usually a weaker demon who doesn’t harm but causes annoyance like banging, moving things etc, - this is what we term a poltergeist. Very rarely a ghost may even be the soul of a damned person from Hell, but this is usually extremely rare, from what I understand.
Now, God may allow a soul from Heaven to appear to loved ones to give them peace, as, for example, He did for me when my mum passed away. The day after her death, I had unexplained happiness, could smell roses all round the house (called the Odour of Sanctity, a sign of Heaven) and even friends in my parish witnessed my mum being around me when I was serving at Mass and afterwards. No doubt, the Lord showed this extraordinary mercy to me for a reason, most likely to strengthen my faith. However, I must stress this is extraordinary, and not to be expected. I certainly didn’t expect it. My mum wasn’t given a Catholic funeral, but an Anglican one (since her family are baptised Anglicans, even though some are converting to Catholicism now!!).
I hope this helps.
God bless!
