Pulling down those branches is an option but not one I would condone. JM can be grown as classic informal upright with sagging branches but that's not really a natural shape for JM. In addition the arc shape you will be able to get on those branches will not even make a good informal upright.
I've moved toward making JM bonsai look like Japanese maples. The natural shape for JM is generally a giant shrub with large branches that sweep up and out so they are almost trunks but the ends swing outward to give the foliage pads we associate with informal upright branches. These trees become a sort of cross between multi trunk and informal upright.
This tree already has a good start with a couple of low branches growing upward. Instead of pulling them down juts take them out away from the main trunk but still going up. As they grow and thicken chop to activate new shoots which will give good changes of direction and much needed TAPER. Gradually build up more and more secondary and tertiary shoots to make up the foliage pad at the end which will sweep outward and horizontal.
Do some online searches for Japanese maple bonsai to find better examples of this style. Meanwhile here are a couple of my primitive attempts so far
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The grower has done you a favor chopping the leaders though they have not been cut low enough IMHO. Creating good movement AND taper requires regular and repeated chop and grow. First the trunk then main branches and finally secondary branching. It appears to be the only way to make good looking bonsai.
All the chopped leaders will grow again from lower nodes. They should all grow just as strong as if not cut and will thicken just as much as it they had not been chopped.
I do agree with your treatment of the top of the main trunk. Cut at a fairly steep angle to start the thicker trunk blending naturally into the new leader. Allow the new leader to grow free for a year, maybe even 2 years until the new section looks right then chop again just 1 or 2 nodes higher.