Why don't you want to use paragraph returns? And why would you use soft returns? As you have seen, you can't use soft returns in justified text, and in a general matter, you should avoid to use them anyway.
If you use soft returns to send a part of text to the following line, don't do this. Instead, use the “no break” feature to keep together the words you want to stay on the same line, and better, use a character style to do this.
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