True.
The problem is this goes both ways. You can find yourself pointing your fingers while others will point their fingers to you that you are the one "adding". The problem is everybody thinks their understanding is the understanding and the definitive Islam. That of course causes the clashes and the rivalry you see across the entire Islamic world which is and has been torn by differances right from the end of the Rashidun Caliphate and internal conflict had began within 20 years of the death of the prophet [pbuh].
Lesson here for all is to not foist our renditions on others and be accomodating. Leave the judgement to the hereafter. Use all your energy to introspect and improve your self. After all you will be only accountable for your own actions. Not others.
I don't think they would be able to. This isn't some sort of ego reaction either, I really only do the basics currently, it would be extremely hard right now for "them", whoever that is, to accuse me of doing any adding anything.
I'm accommodating but there's a limit. A person from a different school of thought who has his hands by his sides during prayers - okay I'm fine with that. However if your worship has evolved to spinning in a circle then I'm going to say that's wrong (but of course in a better manner so it's more persuadable). There are accepted differences of opinion but not everyone comes under that.
No we Muslims are not meant to live like that. We are told to enjoin good and forbid evil. If people are doing bad things, including changing the religion to suit their own desires, then this is something that needs to corrected. Islamic is a public religion too, sure you do acts of worship in private but it is very much public too. People have no problem trying to improve things like education, GDP growth, (less) corruption etc... But they then they neglect their religion in society when they should prioritise this first and foremost.
On the authority of Abu Sa`eed al-Khudree (may Allah be pleased with him) who said:
I heard the Messenger of Allah (ﷺ) say, “Whosoever of you sees an evil, let him change it with his hand; and if he is not able to do so, then [let him change it] with his tongue; and if he is not able to do so, then with his heart — and that is the weakest of faith.”
[Muslim]
Bad things happen when good people don't keep the bad beliefs in check. It opens the door all manner of outside beliefs into Muslim societies.