I understand my options are either using my phone as a hotspot or (dropping the Samsung tablet and) adding a USB wireless connector to a laptop. What are the advantages of each?
An external adapter (assuming it accommodates a SIM card) would not have the restrictions that tethering from a phone would entail. See your plan details for the bandwidth limitation on tethering. The laptop needs to be in WiFi range of the phone in order to tether. Also, if your phone dies or is otherwise nonfunctional, your tethering capacity goes away with the phone. A SIM card based solution would be better, but you'd still have to pay the monthly fee.
I have a Samsung tablet that I pay a $20/month fee for 2 Gbit data service on a Simple Choice plan with 3 more voice/data lines. I almost never use the Samsung, and no one on the plan comes close to breaking their two gig limit.
Does a hotspot use peer-to-peer networking? Can a hotspot be secured so that only I can use it?
It uses NAT/PAT, not peer-to-peer. Yes, it can (and should be) secured.
I suspect that if I use laptops, I will use far more data than I do currently. What options do I have?
Again, refer to the tethering limit on your plan and/or explore other plans. If you will use a lot more data, keeping the SIM from your Samsung tablet might be a better idea.