Hi. As Catholics, we do not view Christians in heaven as "dead" simply because their earthly bodies have perished. That said, in Rev 5:8 the elders in heaven intercede for Christians on earth. In Rev 8:3 angels offer the prayers of Christians. In Rev 6:10 we are told that the martyrs in heaven are aware of things on earth. In Lk 15:7,10 Jesus tells us that there is joy in heaven over the repentant sinner, another reference that saints (holy persons in heaven, if you prefer) are aware of things on earth. And in verse 10 Jesus confirms this about angels too. Paul also indicates the angels are aware of happenings on earth, as he charges Timothy to be obedient in the presence of the elect angels (1 Tim 5:21). While I'm not a believer in proof texting, I do believe there are ample verses to support the Catholic view that angels and saints are aware of events on earth. You are, of course, free to disagree.
Let’s run through your verses, since not even one teaches us to pray to departed Christians - or that the can hear our prayers:
Rev. 5:8 - prayers to God are offered as incense. It does not say any of those prayers were offered or heard by the 24 Elders.
Rev 8:3 - ditto- angels are fulfilling a priestly role - it does not say the prayers in the form of incense were prayed to angel or that angels hear our prayers.
Rev 6:10 - martyred Christians cry out to God for justice - no indication is given that they watch what happens on earth, or pray for others. In fact, they are crying out for God to carry out judgement.
Luke 15:7, 10 - ah yes, rejoicing in heaven over a sinner who rejoices! Beautiful. Yet no mention is made of Christians in heaven. Nor is this indicative that anyone prayed to a departed Christian or that their prayers could be heard.
I Timothy 5:21- again, no evidence that angels are to be prayed to, or more importantly here, that any departed Christian is to be prayed to or can even hear your prayers.
We do know that God hears our prayers and that we can enter before His throne in boldness to petition Him.