[SDPhantom]

Haven't tested it, but this is what I came up with.

/focus [@focus,nomod:alt,help,nodead][@mouseover,help,nodead][help,nodead]
/clearfocus [mod:ctrl]
/cast [@focus,help,nodead][@player,nomod:ctrl] Soulstone

Here's the idea:

/focus

1. [@focus,nomod:alt,help,nodead] - If focus exists and not holding alt, preserve focus
2. [@mouseover,help,nodead] - Fallback to mouseover if exists
3. [help,nodead] - Fallback to target if exists

/clearfocus

1. [mod:ctrl] - Clear if holding Ctrl (overwrites previous line)

/cast

1. [@focus,help,nodead] - Cast on focus if exists
2. [@player,nomod:ctrl] - Fallback to player if not holding Ctrl (prevents casting if you just want to clear focus)

The UI code for /focus is weird in choosing between the unit in a macro condition or its parameter. It may require further tweaking.