Answer: Yes mind is the only means of devotion, physical senses of the body can help in engaging the mind in spiritual practice but it cannot help one attain the fruits of their devotional practice. The spiritual law of God is that you will attain only that which you are attached to - whether it be spiritual or material, it will be according to the attachment of one’s mind, and not according to one’s physical actions.  Thus, if we are physically engaged in worldly activities, but keep our mind attached to God, then we will not have to bear the fruits of our physical actions. We will only receive spiritual results if we can develop a loving attachment of our mind to God.

Answer: Devotion is done daily by doing "loving remembrance".  Just remember God daily "in a loving way" while working, while doing chores or in your free time and you will be doing your daily devotion

Answer: Devotion is devotion.  It  makes no difference if done in a group or alone.  If your mind is involved in remembrance of Krishn, it makes no difference if you are doing alone or if you are with other devotees, because devotion is very personal. In our scriptures, meditation is described  as done in isolation. 
Whatever way of devotion is convenient and possible for you is fine.  However, when there are devotees doing meditation together, they all have their own vibration of devotion in the atmosphere and sometime this can help to uplift another devotee.  So both  ways are equally good, either doing alone or doing in group.    

Answer: Surrender means coming to a state of doing nothing... when you can give up your own ability to do things  and remain in complete acceptance of whatever outcome that may yield. For example - As long as a newborn baby does nothing, the mother does everything for it. When the child starts doing a few things, the mother lessens her responsibilities to the same extent. When the child begins to do everything, then the mother does nothing. This example fully explains what surrender is.
We must be like a day-old child..., and surrender our mind to God and Guru.