Yes, that's mostly correct, but not quite. The God-honoring criminal crucified next to Jesus is unquestionably in heaven. A baptized baby who dies prior to the age of reason is definitely in heaven. Jesus' Mother Mary, as Kecharitomene, is certainly in heaven. We could confidently ask St. Dismas (as he is locally called) or Alfie Evans or the Blessed Virgin Mary --- or the Holy Angels in Heaven --- to join us in prayer.
But you are right about most "folk canonizations" being presumptuous, even if innocently so. "Your Grandma is in Heaven with Jesus now", spoken with such good will, nevertheless makes an assumption which cannot be verified until we are in heaven, ourselves, to see: unless God sends an unmistakable sign.
That's why the Catholic Church has historically required two miracles, after death, to verify a canonization, in addition to evidence of a life of heroic virtue. That's God's sign. It can't just be based on Grandma's reputation for sanctity, which, as you wisely say, might not be the case.
That's God's sign.
That brings to mind a number of scriptures that are written for our admonition:
1Sa 16:6 And it came to pass, when they were come, that he looked on Eliab, and said, Surely the LORD'S anointed is before him.
1Sa 16:7 But the LORD said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart.
1Co 1:22 For the Jews require a sign, and the Greeks seek after wisdom: 1Co 1:23 But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;
1Co 1:24 But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.
Mat 12:38 Then certain of the scribes and of the Pharisees answered, saying, Master, we would see a sign from thee.
Mat 12:39 But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:
I don't know that I'd trust any signs...Jesus says we are to walk by faith, not by sight...