In our studio we layout both RtoL and LtoR languages. We have recently switched across to CC and have installed the App for RtoL languages, this has skewed a number of things on the Macs. Is there anyway of switching between the versions or altering the settings so App does not interfere with normal work (eg Lorum Ipsum appears as Arabic text or a PDF is created without pages be set up RtoL).
It's not necessary to switch back to a non-RTL version of InDesign for the things you mention (or for any reason, really). Both versions are actually identical under the hood, it's just the UI that's different.
So, to set the document direction, got to File > Document Setup.
To change the language of the placeholder text, press Ctrl when you click the menu item "Fill with Placeholder Text", and choose your language. You can probably choose English as your default if you do this with no documents open -- haven't tried that....
As to your question of whether you can have both the ME and non-ME versions of InDesign installed at once -- I'm not sure. It sounds risky to me.
Looking at your requirements, the best suggestion is to use the Arabic (ME) version of InDesign wherein you can choose the "Text Direction". It would practically not possible to have both ME and non-ME versions of InDesign installed at once. One workaround can be to have one CC 2015 and the other one as CC 2017. However that would create more complexity rather than helping your situation. This link can be of help: Find and install Hebrew or Arabic editions of InDesign CC or Illustrator CC