Ok I will try answer in order.
Thanks for wishes and same to you and your family! I am happy you like and enjoy the mission. Sure I am interested in your idea. Make all work in MP was main dev time eater. My next project will be pure SP. Sent your stuff to <ataribaby AT seznam DOT cz>. I will try take a look.
Yes it is playable as SP in same way as in MP - Save option is disabled on purpose. It mimic MP feel - there is no load/save/retry. You can simply abort mission in your hideout and your progress will be saved.
There is Situation/How To Play section in Briefing to get basics how it works and also there is 3D icon help in-game. It is written with very bad English as I am not native and so well on that. Currently all texts was sent to form/grammar English review so I hope it will improve.
Play flow is like this: (it resembles a lot Tarkov from where I gathered a lot of inspiration) You are surviving NATO soldier in besieged town where is other friendly/hostile NATO soldiers and hostile guerrillas. All trapped in town by CSAT for some strange reason. All sides warring for supplies and gear. Your task is just survive as long as you can. There is some small lets say "mini games" that you can discover. Trader is one of them.
At beginning you leave your hideout and you loot town for food cans and gear to live another day. Don't get killed in process and search town for possible exits that leads back to your hideout - it is never spot where you entered town. You need find one.
If you are caught out in town when raid ends you are considered MIA and you start new NATO survivor life as he just found hideout after your dead soldier. Same for getting killed in raid - you will be considered as KIA. Every new raid you will consume one canned food. In SP every mission start is considered as new raid. In MP you can return to same raid after disconnect in your hideout till raid end. Then new one will start and it will cost you food.
You can exit mission anytime in your hideout - your game is saved already when you start mission again time passes and you are ready for new raid. MP works exactly same. I made custom save system that runs at background if you are in your hideout. Just try it - start mission and do not leave hideout. Open inventory and do some changes like drop something and then exit mission and start it again. Save progress is seamless in SP in same way like in MP.
Mission is meant to be hard and unforgiving. Every successful raid should be achievement and you should feel sorry every time you die. It is hardcore on purpose. There is not much story to follow except that back story so you can easily pop-in for 15 or so minutes to get quick tense action and then pop-out without worrying much beside to stay alive. You can set diffetent goals for you. Stay alive as long as you can, kill count, highest score, best gear, do not kill any friendly NATO survivors and so on. It is up to you how you approach game and what story you will make.
DLC is not needed - just don't pick up that stuff and try another. Sorry about that but I wanted some stuff from DLC to be included. It doesn't prohibit to play mission.
Watch those videos where you will see two raids. One successful and one ill-fated.
Here is one of minigames
And here is some AI features showcase
Hope it helps