Mules are tough and also very smart. If they think they can get away with bad behvior they will keep doing it. In some ways they are harder than horses to work with. Being strict and sometimes tough with a mule will not hurt them. They know the difference between tough, strict training and abuse. Once trained they are wonderful creatures and very loving.
Listen to
@Mini Horses, Baymule and Sage Hill. They all have lots of experience training horses and mules. They are willing to help anyone so take advantage of their advice. To reiterate what everyone else is telling you -
1. Defiitely
castrate him - you don't want an uncastrated male even though mules are supposed to be sterile hybrids. Castrating him will calm him down and if you leave him an entire you are asking for trouble in future. A mini mule is cute and lovable looking but stallions can be very mean and dangerous. Just because he is small (mini) doesnt mean his temperament will be sweet and docile when he is grown if you don't castrate him.
2. Pen him in a small -
very small - area where he can't get away from you. If he is in a very small pen it will be harder for him to kick out too. By keeping him in a small area you can work on haltering him, grooming him, teaching hm to lift his feet, etc. He will eventually have to meet the farrier to get his hooves trimmed and those guys won't bother wth bad behaved animals. To avoid getting kicked, stick close to him - right up against him - when walking round him. It will be harder for him to get a leg up to kick if you are right up against him. Also talk to him as you move around him to make sure he knows it is you and not something scary.
3. Once you can get the halter on him and he associates it with good stuff like treats or grooming, you can walk on leading him around. He is a baby and will be very frisky, so you need to teach him to be obedient on the halter asap. He looks small and cuddly now but even at this age and size he is more owerful that you.
4. Remember all his interaction with humans has only been when being fed with the rest of the herd of 34 horse/mule pairs. He knows horses and mules but humans are still somewhat odd to him except when they were fed.
Probably not. What looked overly forceful to you may not have been as forceful as you think. If Maverick was frightened and resisting getting out of the truck the seller may have needed to use more strength so it might have looked more harsh. He is little and cute but is also strong and powerful for his size. You will have to use your strength and leverage when working with him. You don't have to be mean, but do have to be strict and consistent. Once you have him trained to a halter and lead rope you can train him to get in a trailer by leading him.
BTW, we have a lovely gaited mule who will not load into the trailer with sweet talk. Her load command is "D**n it Josie get in the d**n trailer!" LOL She is a mule - that is all that needs to be said.
I was trying to be gentle with you. 
In person you'd hear a really direct set of directions whirling, with halter in one hand and other arm around his neck!! Always easier face to face, in person. But we're not. 
