Mechanically I don’t think anything changes with the number of players, since you always have 4 player characters no matter how many players.
I personally don’t think it would be as fun solo. You would have more control and precision which might appeal to certain people, but for me the chaos of having other people doing things and having to negotiate a plan where everyone is constantly inputting was part of the fun.
In the stealth section there are static guards and patrolling guards. At the bottom of every turn the players pull from a deck of cards which says which of the patrolling guards will move and also a special event- this can be the meter towards the alarm ticking down, some of the guards reversing direction of their patrol, or reinforcements prestaging just off board.
During stealth if a dead guard or a player character is spotted by a specific guard, it will shout alerting other guards inside a certain radius and act according to the combat logic. At this point the stealth section will likely shortly end because of all the negative stealth modifiers.
In the combat section, enemies will move towards and fire at whatever spotted player character is nearest. The combat is very simple, which is balanced by it being very difficult for the players to survive, which means you want to delay combat as long as possible.