Have you asked the Physician Scientists Forum here? That's where many MD/PhD students and faculty hang out to straighten out some of the myths you seem to have heard about. You also can check out what is discussed in the APSA.
In short summary:
1) One does not simply walk into MSTP or ask to transfer. As mentioned, while the mechanism exists, the reality is much, much different. It can happen, but PD's know how they manage their MSTP slots.
2) This is a more honest answer.
Would it be acceptable during the MD interview to discuss my interest in translational research with clinical integration, or do they want to hear more about how I want to go into clinical practice?
This totally depends on the school that interviews you and why you want to attend their program. I know of at least one program I view highly that has Ph.D.'s doing translational research and has opportunities to shadow residents in clinic as part of their experience (unless that has since changed).
There are many programs that require a scholar project or a "research year." These programs also tend to have MD/PhD tracks. You'll get your opportunities to do clinical/translational research, perhaps even get a masters in that area.
You absolutely need a clue about what why you want to spend 10 years of your life in MD/PhD world and must balance your clinical and research responsibilities over time. That's the cost of the covered tuition of MSTP. Your success will also be highly subject to your chosen PI and mentors.
The TL/DR answer: we want to hear about your understanding of achieving your career goal in translational research. I have no clue what you mean by translational research "with clinical integration" without a solid example or two from your experiences, perhaps even an LOR.
3) People have done PhD first then gone to MD (Francis Collins, though that was decades ago). Know that the training is totally different, and transitioning from a PhD life to an MD life requires a real change in studying and a lot of humility.
At this point, don't sound inflexible (which is really what will ding you with a knee-jerk reaction). Let us know what you value in choosing a path towards translational research, but understand this area needs a lot of diverse-trained individuals and a lot more people who can navigate the infrastructure and bureaucracy.