The Glyph of Shadowbolt is neither a DPS gain nor a DPS loss over time. It takes your main Shadowbolt spell and breaks it into 3 smaller components, each with 1/3 the power of the original, so that when they all hit, you come out to your normal Shadowbolt. Over a small amount of time, you may see the following scenarios:
1) Shadowbolt hits - 100% damage
2) Shadowbolt crits - 203% damage
1) 0/3 bolts crit - 100% damage
2) 1/3 bolts crit - 134% damage
3) 2/3 bolts crit - 168% damage
4) 3/3 bolts crit - 203% damage
Now let's assume a 25% crit chance in combat. Whether that is higher or lower than yours shouldn't matter in proving how this glyph affects your DPS.
1) Shadowbolt hits - 75% (100% damage)
2) Shadowbolt crits - 25% (203% damage)
Overall damage - 126% normal damage
1) 0/3 bolts crit - ~42% (100% damage)
2) 1/3 bolts crit - ~45% (134% damage)
3) 2/3 bolts crit - ~11.5% (168% damage)
4) 3/3 bolts crit - ~1.5% (203% damage)
Overall damage - 124.6% damage
The numbers listed with the glyph where simulated in Excel with a discreet, independent probability of each Shadowbolt critting (25%) independent, but the overall percentages were built on each independent case coming together dependently. The numbers will change slightly each pass (I got anywhere from 123% to 128% damage) of the simulation, but you can see that overall, you will see nothing except an aesthetic change to Shadowbolt with the glyph.
Trinket proc rates are not dependent on the NUMBER of spells that hit. As long as you are always casting something, the more important note about trinkets is their internal cooldown. There isn't a trinket in the game that behaves based on the number of hits received. Light of the Cosmos does not proc more often for an Affliction Warlock (3 DoTs) than an Elemental Shaman (1 DoT). They are based PURELY on their internal cooldown.