Territorial/resource guarding dog (6yo) — is facility-based training the right approach, or should we insist on in-home sessions?

Our 6-year-old dog (10kg cavalier + poodle mix) has significant territorial and resource guarding behaviours specifically in our home when guests visit. Barking excessively for 10+ mins on entry, unsettled and barking again when visitor makes sudden movements.

We've consulted a trainer who has recommended starting with 7–10 sessions at their training facility without us in the room at first, with the possibility of up to 20 sessions total. Her reasoning is that his behaviours are so ingrained that she needs to first "undo" them in a neutral environment before teaching new ones. She has said that she may suggest home visits external after the booked sessions are up, if she believes she cannot progress further.

We're committed to doing this properly. Money is set aside and we know it won't fix itself. We just want to make sure we're spending it wisely, so a few questions for anyone with experience.

1. Does facility-based training make sense for territorial/home-specific behaviour? Our concern is that he won't exhibit the problem behaviours at a neutral location, so how does the trainer actually work on them? We do understand there will be homework and we will be doing training at home with him.

2. Would in-home training not be more effective here? It feels like training him in the environment where the problem occurs would be more targeted — are we wrong about this?

3. Is 20 sessions a reasonable estimate for ingrained anxiety-based behaviours in a 6-year-old dog? We want to be realistic, but we also want to make sure we're not over-committing before seeing any results.