As I said before- it should just be a matter of easing the entire root ball out of the pot to have a look then put it right back in again. Even if you have to lay it on its side and give the rim of the pot a few thumps, the whole root ball should just slide neatly out and retain its shape.So I think I'll make a move to take it out of the pot. Its quite heavy and unwieldy but I'll think of something... Can I do it at this time of year? And if it turns out the roots look bad, what should I do (can I root prune the bad bits off this time of year?).
Unless you find something drastically wrong I would say just slide the whole root ball back into the pot and wait for winter to re-pot properly this time. I have found it is possible to root prune tridents in summer but most will advise against it. Unless you are really confident and experienced it is far better to wait for winter.
I believe even more strongly now that your tree is just starving. It has not had anywhere near enough nutrients for such a large tree. I would not even bother looking at the roots - just feed it with liquid fertiliser for the rest of this season.
Don't worry about the wooden block now. I don't think it will be doing much harm. The bonsai mix will be fine for a tree in a pot like this.
I would recommend a quick acting soluble fertiliser for now. Thrive, Powerfeed or similar will give rapid results. Use either every 2 weeks until the tree loses leaves for winter. I normally alternate between several different fertilisers so my trees get a good range of nutreints and trace elements - Osmocote mixed in the potting mix , Thrive, Powerfeed, Dynamic lifter (treeman will object to that too), worm wee and anything else that comes along.
Salt toxicity is really only a problem when you don't water enough. You have told us that water runs out of the bottom when you water so that's good. the salts are mostly water soluble so they get leached out of the mix when the water runs through the pot. So do the nutrients which is why potted plants need more frequent fertiliser to keep them healthy. salts are mostly a problem when you don't water enough to wash them out of the pot so they accumulate and can reach toxic levels.
I still believe your tree is just starving. Feed it.
Reverse taper is only a problem when you have several strong branches growing from one point. From the little I can see of the first fork I think your tree will be ok when it is pruned properly. that depends what style you are planning with the tree. I prefer tridents as informal upright trees in which case you will be pruning everything above the fork except for a short piece of one of those strong branches. If you want a broom style trident bonsai then some reverse taper where the major branches come from the trunk is natural and unavoidable - don't worry about that yet.
