1) Are they telling you when you need to respond by? If not, then just say that you'll need more time or delay it by negotiating more. If they want you to sign sooner than you are comfortable, then you can negotiate a higher sign on bonus to make you more comfortable if this is acceptable to you and your partner. If you want to be less confrontational about needing more time, you can set up meetings with them to continue the conversation to negotiate and schedule those meetings weeks out at a time to delay the decision. If they're looking to expand, I highly doubt the opportunity will close even if they hire someone else. You're directly generating revenue more than the expenses unlike other non-clinical staff members in clinics.
2) Come from a place of strength and say how your additional skills and training are going to bring a unique aspect to your contributions to the practice. What they probably care about is profit. If you can support how these additional skills are going to contribute to higher earnings for the practice overall, then it's a stronger argument. You want to communicate how you can translate your skills into marketable services. This can be in the form of a unique niche to capture certain patient population that hasn't been tapped into yet but has high demand, contributing to the culture of education and collaboration, the growth potential of potentially starting a specialty area that you can build up in terms of different programming (IOP, PHP, RTC in specialty areas are highly desired because you can charge facility fees and then it becomes more lucrative, especially if there's real estate equity involved).
If you do #2 well, then you will also address #1 as the back and forth conversation takes time.