I got my hands on this multitrunk japanese maple, and need some advice and ideas for future designing.

The first thing that comes to mind is that it has 4 trunks, 3 of which is dead straight, has nearly same thickness and with no movements. My idea is getting cutting one trunk of in the winter, so we have an odd number of trunks which is preferred with multitrunk bonsai. I am unsure which one to cut, and very afraid about the scaring - as seen on the picture the cutoff of one trunk by the previous owner is very clear and very easy to notice.

Second thing is what I should do about the equal thickness and non existing movement of those trunks. Can threadgraft at a lower point and the regrow the trunks be a solution?

If the plant is healthy, you can also decide to cut back, without a graft. Japanese mapes will backbud profusily after heavy trimming.

There is nothing you can do about the absense of movement in the trunks this size, except cutting back. As it is a fairly young plant (No real corkbark formed yet) you can probably get it to heal over without too much scarring.

Before you act, make sure you know what your gaeplan is with the final tree in mind.

Whichever trunk you decide to remove, early to midsummer is best as maples will heal more quickly at this time of year. Make a neat, concave cut and seal with cut paste to promote healing.

without a clear view of the entire tree, it's hard to advice