If combat is initiated, EVERYONE rolls initiative and enters combat. Combatants who were unaware are Surprised and cannot act in the first round.
There's no need to have everyone Ready attacks before starting combat -> this is covered by the existence of the Surprised Condition (effectively giving the party a surprise round if the enemies fail their Perception checks).
If a single enemy finds a stealthed party member, then I'd argue that only that enemy is automatically aware of the party. Other nearby enemies would still need to succeed their Perception check or be Surprised, making it very unlikely that you'd have situation where every single enemy can attack that PC before that PC (or the rest of the party) gets a turn.
The way I am seeing it is that if all characters are brought into combat at the same time, then only groups of four players will be able to make surprise attacks since they are only controlling one character each and can coordinate properly using chat, just like in DnD when a group all ambushes at the same time. The attacks are all happening at once, if the enemy is unaware of the party, then they don't get to make any extra checks or roll initiative in the middle of the ambush.
Not sure that Larian would get it right in singleplayer to have all four of our characters get to make their surprise attacks before the enemies do anything. Once the first character has attacked, there would have to be an immediate popup or something asking if we wanted to do surprise attacks for each of our other characters (to simulate the full party ambush.) I feel like their version of Surprised would be the enemy constantly rolling Perception checks in real time until our characters who are now in TB mode are guaranteed to fail their stealth rolls, and because we cannot control more than one at a time, we get screwed out of a proper ambush. Having an actual pause and ready attack features seems like it would be less complicated than all of this.
