Why can’t I fit five GCDs in Blood Weapon (or Delirium)?
The single most likely reason is that you aren’t weaving Blood Weapon late into the GCD. As long as you are weaving Blood Weapon late and keeping the GCD rolling during it, five GCDs in Blood Weapon should be very consistent. With high latency (100+ ms), double weaves can cause the last GCD to fall out of Blood Weapon. It is possible to mitigate some latency by using a gaming VPN (for example, Mudfish or PingZapper).
I’m disengaging from the boss, should I use Unmend?
Generally no. Breaking a combo by using Unmend in the middle of it is a bigger loss than waiting to get back to the boss. You can safely Unmend if you are leaving the boss for long enough that your combo would drop anyway (15s or roughly six GCDs), or if you are between combos and disengaging for at least one full GCD.
Should I hold Blood Weapon for Delirium? Should I hold Delirium for Blood Weapon?
No. There is no inherent synergy between Blood Weapon and Delirium—Blood Weapon will always give 3000 MP and 50 Blood, regardless of if you use Delirium at the same time. Unless you know your kill-time and that holding one or the other will not lose a usage, do not hold either Blood Weapon or Delirium.