Small signal model of Boost DCM
When I am working on these complicated circuits, I always to try to simplify equations and rearrange the whole thing in a friendly shape. This is what I've done from the founding circuit shown below:

The current source connected between terminals a and p includes a reference to \$N\$ which involves the DCM duty ratios \$d_1\$ and \$d_2\$. I thus would like to replace this source by a single equation without reference to other sources. In this expression, the current monitored by the dummy source VIC is actually current \$I_c\$. If I manipulate equations for \$N\$ and \$d_2\$, I have:

If I now multiply this expression by I(VIC) or actually \$I_c\$, I have:

The voltage \$V_{ac}\$ is \$V_{ac}=V_{(a)}-V_{(c)}=-V_{(c)}\$ because terminal a is grounded. Therefore, the final current source is:

You could also remove the negative sign and revert the current source, it is the same. All these models are explained in details in my new book describing small-signal models of switching converters. Going through the details and learning how the transfer functions are derived is the best way to understand loop compensation later on: you're on the right track!