Where I am it hits about 80f in the house most days even with the air conditioner on, I have a dark place in the room that is the coolest, but I know the room never drops below 25c during the day. Is that too warm?
My environment sounds a little bit kinder. I doubt my linen closet ever gets above 70F/21C. Does it make a difference? I am not smart enough to offer any sort of an intelligent opinion.
The nic I put into 15ml & 20ml LDPE and just pull one out a couple of hours before I need it.
The PG and VG stay in the 1 gallon jugs; and I just fill up 8 ounce dispensing bottles as needed. I am not concerned about headspace.
For nic, on the other hand; I agree that minimizing oxygen exposure, by controlling headspace, is a prudent idea. My nic (other than the current working bottle) is the only DIY component I keep in the freezer.
LDPE plastic is probably the least ideal choice of container (for long-term storage); as it is the most permeable to air (of LDPE, HDPE, and PET). PET isn't recommended for repetitive use. HDPE bottles are not easy to find. Glass is the most popular choice; because it is easy to find, comes in all popular sizes, and when properly capped, is nearly impervious to air. For future nic purchases; I would suggest considering glass.