Good to see you trying wollemi as bonsai, quodl.... They can be grown as bonsai, though you may need to give some more thought about styling them. Try to go with what the species offers best, rather than starting with an idea of a shape and trying to fit the species into it.
Here is pic from the VNBC AABC Conference in May this year. It is of a wollemi and is well done. No special 'style' was in mind of the artist, but just some successful experimenting.
The wollemis are really very different from other conifers. The side branches have the leaves, but the tree naturally drops the whole side branch, rather than individual leaves, when the branch becomes too old. Tip pruning the side branches can stimulate ramification, with one or two new branches from just behind the cut. This does not always happen.... It occurs naturally with a very small proportion of branches in trees in the ground and over 10 years old. It also has been achieved with bonsai in pots.
The trees are known to produce new trunks when the leader is cut back, as well as before the leader is cut back. This happens in the wild quite regularly, as well as in pots. It produces multi-trunked individuals, so that can be built into your thinking about styling.
I wouldn't worry too much about the minor reverse taper you've seen. The tree is very young and the trunks thicken quite well with a few years behind them. The bark becomes 'bubbly' and helps mask the apparent issue.
I'd like to see where you have got to with your tree so far

.
Cheers
Roger
You do not have the required permissions to view the files attached to this post.