Dorico Pro 3.5 will not sound an A clarinet playing a low E

[pianoleo]

HALion’s full of these sorts of things. Either switch to GM patches for the instruments that have gaps, or invest in a library that covers the patches you need. It’s clear that HALion won’t ever be rectified in this regard.

Extending Cello Playback Range?

The ranges of the instruments in HALion Symphonic Orchestra were set some 15 years or so ago when that library was recorded, engineered and programmed. There’s zero chance of the HALion sound design team returning to HSO now to change these patches, I’m afraid. Either use the GM cello sound, which covers the whole range (but which doesn’t support techniques like pizzicato etc.), or use a different library that provides the range you’re looking for.