Very true...And at the same time we can not know who IS in heaven...Anyone can look and sound religious but only God knows what's in a person's heart...
That's why it is not wise to pray to anyone except God...That person may not be where you think he/she is...
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.