Retreating root canals are a common procedure. Typically we need to widen the canals out which could lead to weakening the tooth in some cases. Sometimes the retreatment does not work and either an apicoectomy might be suggested or and an extraction. Please note that most root canal failures I see are due to leaky crowns which allows bacteria back in the canals. Ask you dentist of the age of the crown, and should you replace it.
Any time you put off a filling you are asking for trouble. They will continue to get worse and worse over time, possibly causing the need for a root canal, or even extraction due to not enough tooth left.
1) it might not work
3) pain and swelling, bruising
4) depending on where it is being done, you can get a sinus perforation, or damage to the mandibular nerve.
Make sure you know of the alternative of retreating the tooth. 99% of the root canals I see are treated with retreatment first. Apicos are last ditch efforts to save a tooth. Most failures of older root canals are a reinfection. An apico does not deal with that. In general, the apico is only done when the dentist thinks there is a secondary canal at the root tip and they want to cut it out.
Being a diabetic, you are at risk of not healing up as well. The less controlled diabetic you are, the higher the risk involved.