Reaching out to SunRail is the easy part. Actually getting your County to cough up the financial commitment to be contributed each year is the not so easy part. At present Volusia is trying to hard to weasel out of the agreement that they entered into. We'll see how wel Lakeland officials do convincing their county to make the financial commitment. And yes, it is the County that has to commit funds over the long run since FDOT won;t fund it beyond five years for startup, if that.
It is higly unlikely that Amtrak will set up an LD train turning facility in Tampa for just one train when they can just cart it over to Miami while providing some intra state service in Florida. Of course, after Brightline has built out fully covering JAX, ORL, TPA and MIA, then I suppose Amtrak could just terminate all its trains in JAX.
They could. Although in that long run, terminating Amtrak at ORL and servicing everything at facilities in Sanford is possibly more plausible.
Brightline is going to take a lot of Amtrak's ridership south of Orlando, there's no way around it. Hialeah is going to start looking like a worse servicing location than Beech Grove, which is already stranded. At some point Amtrak is going to have to think seriously about its bad shop locations.
When Tampa gets Brightline service, if Lakeland also gets service from Brightline or SunRail or anyone, then the Silver Star detour to Tampa will be unsustainable, I suspect. The intra-Florida traffic would be marginal at that point (Okechobee, Sebring, and Winter Haven to Tampa/Lakeland -- appears to be 5000 to 7000 people per year, and dropping each year). And it might actually be faster to take Brightline from Tampa to Orlando and catch Amtrak at Orlando than to take Amtrak direct from Tampa, which would eviscerate the main business.
That'll leave Okechobee, Sebring, and Winter Haven as the only stations between Miami and Orlando which don't have alternative service. These are significant so Amtrak would probably still continue running to Miami until/unless they get alternative service. Which they might -- a quick googling found advocacy from Polk County elected officials for connecting Winter Haven to SunRail.
However, at this point I don't see Lakeland getting Brightline service for quite a few years, so the current service patterns will probably remain unchanged for a while.