Will mostly include the Rebbe's words which comfort only as they can and these are the words that Mendel would want. When dealing with anything this week, have had one thought over and over again, what would Mendel want and how would he go about it? He was and is that kind of person. He is for sure with the Rebbe asking for, wanting, and demanding the best for you.
I remember how he once took me aside on a Gemara to convince me to see something through - when I was not popular and was hard to deal with. He showed caring and wanted to do what was right. I remember a canoe trip when he was my partner on the canoe and I couldn't or didn't know how to turn. He put up with my trying to row, with half of my turns being in the wrong direction. He was amazing, even when very young, in every way.
Know that he influences everyone he ever met, in every way. Know that everyone is now trying to learn and live from Mendel and feels Mendel actively comforting them at this time. Know that all are with you, your parents, and your family and that while tzaras rabim chatzi nechomo - for we are all with you in your pain - the other half is that Mendel is very much alive and in the world and is inspiring everyone. The Rebbe would say clearly that a chosid doesn't pass, goes higher but is the same person, and even Mah Zaro BaChayim, that he actively works, in reality, through others down here, is also by chasidim and not just by Rabbeim. And if there's one chosid in this generation, it is and remains your brother Mendel.
Last year, in my building, one of the Meron Kedoshim passed away. Mendel's going feels the same, a tremendous void that will have him bring down Moshiach and Techiya. Know that he'll be back before you know it. One can feel it.
A few months ago, his cousin Nechemia Plotkin helped my son, who was adjusting to Israel, get into a school. This gave him a lot of confidence. Nechemia told me a lot about his favorite uncle, Reb Mayer and showed me pictures of your father attending his parents' (your father's brother's) wedding. Indeed, Mendel came from a whole tree of kindness and making a better world and he continues to do so from a higher and even more effective level, until he is back soon with techiya.
Am with you, davening for you, and want to give you encouragement. But the words of the Rebbe, the truth of this message, are the only possible comfort. They have comforted many and am sure that they will help you.
True Words of Comfort From http://www.beverlyhillschabad.com/jewish-stories/text/rebbe-next-world.pdf Also printed in the book "To Know and To Care":
Rabbi Nachum Rabinovitz, an elder chassid who lives in Jerusalem, was once waiting for yechidut (a private audience with the Rebbe). Among those waiting was a young man, obviously wealthy, but wearing a morose and despondent expression.
A short while later, the young man entered the Rebbe’s room, and when he emerged, his expression had changed. His face beamed forth energy and vitality.
Curious about this abrupt shift in emotion, when his own yechidut concluded, Reb Nachum inquired about the young man’s identity from the Rebbe’s secretaries and was able to arrange a meeting.
“I am a wealthy man,” the young man told Reb Nachum, “but recently, my only son died. At that point, I felt that my life no longer had any purpose. I saw no value in my wealth or my position.
“I went to the Rebbe for solace and advice.
“The Rebbe asked me what my feelings would be if my son went overseas and was living in a foreign country from which he could not communicate to me, but in which I could be assured that all his needs were being met and he had no suffering at all.
“I answered that although the separation would be difficult to bear, I would be happy for my son.
“ ‘And although he could not respond, if you could communicate with him and send packages to him,’ the Rebbe continued, ‘would you do so?’
“ ‘Of course,’ I answered.“ ‘
This is precisely your present situation,’ the Rebbe concluded. ‘With every word of prayer you recite, you are sending a message to your son. And with every gift you make to charity or institution which you fund you are sending a package to him. He cannot respond, but he appreciates your words and your gifts.’ ”
One time visiting a shiva, the Rebbe explained that no one goes away. A passing is like someone moving from a mundane bayis to a bright aleya/upper floor.
Rabbi Aron Sirota often explains that we use the term "HaMokom" to stress that Hashem who is here with us is the same Hashem who is there with our loved ones above and connects both us and them together.
Wishing you only brochos and comfort and may you know no more tzar. May we see בלע המות לנצח and techiya, an ikar in our das.