Yes, you have a choice who leads your onboarding, whether that is with HubSpot's partner outsourced onboarding program or with a certified HubSpot partner.
Yes. When going for a HubSpot guided onboarding, HubSpot may ask if you want to be part of their partner outsourced onboarding program. If that is the case, then you will have the option to choose a HubSpot partner to work with. If you have a partner in mind, you can ask to go with them.
Alternatively, if you know the partner you want to work with from the get go, then you can go with them, even if they are not part of the partner outsourcing program. A certified HubSpot partner can still onboard you to HubSpot, as long as you sign the documentation for HubSpot to know you are working together with onboarding.