1) For /volume If 1350GB are used on a 3612GB volume, surely the available space is 2,262GB
As you said, no space-guarantees, that means 'aggregate space' availability is key here. Could you check the space usage of the aggregate of the affected volume? You might be running low on space on the aggregate. Let us know.
2) For .snapshot. How can 394% of the total be used?
Yes, it can happen with NetApp ONTAP volumes. Total volume space = Active File-system space + Space reserve. In your case, it is set to '2%' of 3612g = approx. 73G, which means Active Filesystem cannot write into this space (72g), it is strictly for 'snapshots'. However, snapshots can spill-over to AFS. In other words, snapshots filled-up the reserved space and then started to spill-over to the Active File-system. The percentage of 394 suggests, it used 394 % of the total reserved space, ie. 394 * 73 / 100 = approx. 290g. You may want to look at your snapshots frequency/schedule or setup some auto-delete mechanism etc.