Is there any evidence specifically from Shia sources that confirms that the Prophet (pbuh) used to combine his prayers even when not in a state of emergency?
Yes of course. There are authentic evidence in both Sunni and Shia books. You can find many Hadeeths in this regard in Wasaa'il al-Shia.V. 4, page 220. باب جواز الجمع بين الصلاتين دون عذر
You can also find many Hadeeths in Saheeh Muslim, Baab al-Jam' Bayn al-Salatain. باب الجمع بين الصلاتين
Wassalam.