Unless I am missing something, I don't see a second leader.....unless you mean that low branch off to the right. I wouldn't consider that a "leader" and at any rate, Japanese maples do not really have distinct leaders unless specifically trained to, as some weeping forms are. Once they start maturing and branching well, they seldom develop a 'leader' other than whatever trunk structure they may have. Removing anything that may be a considered a leader at this young age is unnecessary unless it has developed an uncomfortably tight crotch angle. If it is that lower branch you are referring to, removal is up to you. Personally I'd let it go for awhile to see how it develops - depending on your specific aesthetic, low branching on JM's is often considered a desirable trait. As to the ID, that leaf is very heavily dissected yet good sized. I'd guess Acer japonicum 'Aconitifolium' or the fernleaf fullmoon maple, an extremely desirable selection :-) What does the fall color look like? If Aconitifolium, it will be on the smallish end of the scale, growing slowly to about 15', give or take. And Japanese maples do not develop invasive root systems - your retaining wall should be fine.

Frozen branch tips on JM's can be pretty common and often are black in color. They will not re-leaf and should be removed. And the stress of experiencing a late frost (or lack of sufficient acclimation) can result in sparse and less than vigorous foliage, typical to what the tree looks like now. That said, these can be remarkably resilient little trees and planted in the ground with a bit of time for establishment, it should improve its looks 100%. It is not easy to tell from the photos but it does not look like its overall form and structure was significantly compromised by its brush with cold weather. If the price was right - your call - I'd get it.

With that much experience, I am surprised your guy suggested October. Especially in some place like CT that can experience sudden and sharp cold spells in winter. Even the ISA recommends dormant season pruning. In addition to other reasons, it makes it far easier to see what and where to prune to get the best aesthetic results when the tree is out of leaf. This link on pruning Japanese maples has some excellent and well-illustrated advice on how to prune to accent/enhance structure or growth habit and how to thin out a congested or too dense canopy. I'm not sure I understand the concept of 'exposing branches' outside of the context of canopy thinning, but perhaps the link will provide sufficient visuals on how a well pruned tree should look.

If your plans are to allow the plant to grow in a container for its entire life, It will take 20 years for a bridge graft to outgrow being anything but an eyesore. If the tree is planted out in the landscape at some point in time, it will become a hazardous tree with a trunk weakened from the inside out. The callus tissue along the left side of the wound is being supported (with water and nutrients) by the thick root immediately beneath the lower edge of the callus and moving down/left. I suspect the same is true on the hidden side of the wound with another root, meaning all the roots beneath the wound are or will be dead and a hollow trunk is in the tree's future. So, if you keep it in the pot you can treat the hollow trunk as something of a feature if you like. I guarantee it will look better than a bridge or inarch graft. Of course, you could easily layer the tree off above the wound and have a perfectly healthy tree. This maple was layered off of it's roots: You can see the tourniquet wire still attached ^^^ under the trunk where the old roots were sawed off. Below is a hackberry being layered ..... ..... and here it is 3 months later after separating the layer: Ask if you need help. Al