Catena 30

Nothing about your set-up sounds dangerous to me. I sleep cold and general prefer a bag that is about 30 degrees lower rated than the temperature. You might wake up a bit chilly in the night. I would add in hot water bottles or hand/foot warmers into the sleeping bags. Wool hats. You can always put on more clothes in your sleeping bag. Thick socks. How are you on food prep? Are you depending on your ability to light a fire, or do you have a stove as backup? Hot drinks before bed help keep you warm. Do your best to not get cold and wet, and if you do, warm up with hot food and clean dry clothes. Hot food in your belly does you the world of good to feel better if it seems cold and miserable. And remember, you can always leave if you want to!

Bring something to make a warm, non-caffeinated drink before bedtime. I like a hot toddy with lemon juice and honey with a shot of whisky. If that's not your bag, herbal tea is nice. You sound like you are coming to this with a common sense attitude, which honestly, I think is what keeps folks safe.

I am somewhat in awe of your prehensile kicking style that has loosened the cap of a hot water bottle! Yes, their set-up will probably be a bit chilly. They might not be comfortable. It's their first time out. They have to decide if they want to spend $$ on a new hobby before they start, or put up with some discomfort. Maybe I'm just cheap. :)

I have a Kelty Catena and it is very slippery inside and out. According to the REI listing it's polyester on the inside and polyester taffeta on the outside.

I would argue against hot water bottles because if they get kicked during the night the lid can loosen and now you have a wet sleeping bag and hypothermia. Sometimes I'll warm up a similarly sized rock. But, really, you need a system that doesn't require playing these kinds of games. And if OP has a 30F bag and air mattress, that's not going to be enough to be comfortable at 40F.

I wanted one too, but ultimately my husband and I each got a rectangular Kelty that can zip together, so we can choose to sleep together or separately, it's less unwieldy to put away. We have so far slept in it zipped together every time though.

Agreed- instead of buying everything all at once, take some time and figure out what you like and get it over the next 3-4 trips.  For car camping, it’s OK to get an entry level tent like Coleman or Ozark Trail. At least a 4 person tent for 2 people is nice and roomy.  There are some good entry level sleeping bags out there- for car camping I like rectangular and flannel lined. I used a basic Kelty 30° bag last weekend and it was cozy and economical. Coleman is also a god choice.  You can get a solid single burner propane stove for $30, or invest in a 2burner stove that’ll last you 50 years.  A lot of other stuff you can figure out or adapt from stuff you have at home already, especially cooking and eating gear.  Have fun- the most important thing is to get out there, not being fully geared up!

I have the single version of this and also love it. Super versatile temp-wise.