Apologies for another beginner's corner question.

I'm trying to do the best for a couple of azaleas which look to me to
be in serious need of some TLC.

Now that they've finished flowering I've nipped off the spent flower
heads, done a careful pruning - there was some towering sort of growth
and I'd like to encourage a more spreading habit - and I'm about to
dig in some ericaceous compost for a feed and a more acid environment
but here I've hit a snag.

One of the plants appears to have been planted quite high in the
surrounding soil and whatever might have been the medium it was
originally planted in is almost impervious to the trowel - it's a kind
of inseparable peaty mass.

Is it ok to dig as aggressively as would be required to penetrate into
it in order to introduce the ericaceous stuff or should I just lay the
compost on top of what's already there?

I'm assuming whatever it is, this growing medium must have been
deployed for it's water holding properties. Is it of any particular
benefit to buy plants sold like this?