Unless I'm missing something here, I think this is a misunderstanding.
what happens if you can freely toggle combat everywhere?
The request wasn't to freely toggle combat, it was to freely initiate it... or more accurately, not initiate combat (we already can) but to initiate turn-based mode. Most of the problems you list have to do with
stopping combat arbitrarily. The other bits -- maps flooded with hostile quest-givers, etc -- can (and do) arise already, since you can force-fire at will.
Some serious questions / oddities / annoyances arise from the current handling.
From a rules perspective:
What's
not exploitative about (potentially) giving a free turn to your entire squad at the start of combat? Select high-initiative snipers, force-fire, combat formally begins, snipers shoot again, and then the target finally gets a turn.
Then (more importantly) from a user perspective:
What is so special about that opening shot, that it, unlike every other shot in the game, is based on real-time click-accuracy in a limited window of opportunity? With the with the small character size, idle-state wanderings, camera rotation, spread-out interface, and the line-of-sight problems... it can be frustratingly common to have the target become unavailable right before (or as) you click.
I seriously doubt one of the system's goals is to provide a brief, mouse-only Crusader-style minigame to reward player skill with extra opportunity.
A cheap solution would be forcing the combat state to start whenever you click the weapon slot... but that makes bashing a serious annoyance. RTWP-style pause-and-target is a decent workaround for the second problem, but not the first.
So let us slap the space-bar when not in combat, and give the opening turn to the selected character. For multiple-selected, give it to the first / left-most on the selection bar (highest initiative would make more "sense" but isn't as clear.) Then we can take that "first turn" at our leisure, AND it isn't completely free / simultaneously taken by multiple characters.
That could allow
actual exploits re: positioning... cheesing your way into cover through the "engagement radius" or even running your melee right up into the back-line's faces. IMO that's a less severe exploit than the seven-gun salute, and more easily squashed. This (one) idea's a bit of a band-aid, but it's logical and consistent...
Say we're in the "pre-combat turn." That is: we're in turn-based, but no attacks have been made, the selected character is outside hostile awareness / proximity. Essentially, nothing has happened that would cause the game-as-delivered to initiate the combat state.
Each character who is hostile-by-default (i.e. if they became aware it would trigger combat) could have temporary "ambush mode," maybe even full-on interrupt to be fair to melee. Or, maybe even more simply / naturally: if you do any (non-attack) action to that violates the non-combat state, that turn stops, and combat starts as if you had done the same thing in real-time.
Maybe slap on another check to squash the seven-gun salute for good: if in RT, and requesting to force-fire, instead start a turn with the "first selected" character... or even have that one character take the shot, and then proceed as if that was the start of their turn, filling in the rest of the initiative queue behind them.