I really should have researched this a little more before the original post.
The same situation in the browser version forces you to click the specific button. The mouse "click anywhere" feature will not close the window. This is what I'm proposing the app version should be like.
I will edit OP to point this out.
Farti
The precise spot in the browser is convenient for the way input on a (non-touch screen) PC/Laptop works.
In fact the placement of this button and the heal all feature was recently the subject of a pretty significant uproar in the feedback section (and the wider forum, Facebook, YouTube, and in-game communities) when Inno edited the battle screen placing the 'heal all' button exactly where the mouse pointer would previously have perfectly aligned with both the 'auto battle' and the 'close results screen' buttons, which was one of the main appeals for rapid fighting on PC and was, as a result of changing the placement of the heal all button, turning into a lot of unnecessary diamond expenditures and loss of momentum for highly active fighters in the PC version.
The convenience factor of 'being able to click anywhere for expedience' doesn't apply on the PC because there is only one point of input.
You can't quickly tap with your left thumb so that you can keep your right thumb in position for your next move (as an example).
Moreover, as said in reference to recent changes and resulting uproar, the 'exact button one has to press on PC' has been placed so as to be convenient for an environment which uses this singly point of input (the mouse pointer).
On any smart device the user can 'tap anywhere' very quickly but most people require muscle memory for specific sequences of precise clicks. The more precise clicks a routine contains, the less likely people will truly master it or even bother trying and so the less likely people are to become effective speed fighters on a mobile device. Therefore, the larger the area for any tap, the better and the more different areas, even better as it then allows preferential routines for different dominant hands (another issue one does not have on PC where the dominant hand does -all- the clicks.
Now, so as not to restrict myself to raining on your parade...
One way that could potentially achieve what you want, without impeding the potential processing speed of everybody else, would be to approach the issue from a different angle altogether:
What if units that are killed in battle remain visible in the 8 unit 'active' army UNTIL they are removed/replaced/healed/another battle is commenced?
Instead of changing how the battle screen is closed, change the effect closing the battle results screen has on 'permanently losing killed units'
Make it so that when the battle screen is then re-opened, the 'heal all' button is provided and any 'dead' units are shown either in red (as they do in the battle results for the previous battle, where they were killed) or in 'grey hashes' or some other clear marking to show these units are 'dead' and therefore won't join battle if battle is started with the current line up.
There are then five options:
1) The user could then 'remove them from the line up', causing them to disappear;
2) The user could 'drag another unit into the line up', causing them to be 'replaced' or 'overwritten' in the active army and thus to disappear;
3) The user could 'heal specific units' causing those units to show fully healed in the line up (and able to be removed from the line up without being lost)' while allowing other damaged or killed units to be removed, replaced or ignored;
4) The user could 'heal all', replenishing the entire line up that was used in the previous battle; or
5) The user could 'go to battle' or 'auto battle', leaving damaged units damaged and causing 'dead' units to disappear.
Having this 'secondary opportunity' to heal your units would not impede in any way on the routine and battle speed of people who do not wish to use this feature. And to top it off, there could even be a 'no-heal/kill confirmation' tick box (similar to diamond spend confirmation), which a user who wants to use diamond healing can activate so that a THIRD opportunity (a warning pop up) is presented if that user tries to do any of the above options 1, 2, or 5 where that action would result in the permanent loss of a unit, providing far more of the 'loss prevention' you seem to want while not actually affecting anybody who has no interest in that protection.