A framebuffer-based GPU doesn't generally do much to help you emulate a scanline/tile-based VDP/PPU-type processor (aside from things like scaling/filtering the rendered frame).
Aside from using it as a simple blitter, no. It's no help at all. Genesis Plus uses the Dreamcast's 3D hardware as a blitter, and it's very fast. However, since it can only draw whole tiles, and there's no way to map the Megadrive's VDP effects to the PVR's drawing operations, there are lots of things that it just can't do.
The SNES PPU is far more complex that the MD's VDP, and a larger proportion of the available games use the extra effects or drawing modes. In principle, you could use the DC's PVR as a blitter in a SNES emulator, but compatibility would be far worse than it would be for a Megadrive emulator.
You're talking about an interpreting CPU core. I'm talking about a script 'interpreter' ala SCUMMVM. I assumed that's what the OP was referring to when he said that bleemcast was more akin to an interpreter than DSNES is.
Not really. Bleemcast is a proper PlayStation emulator. It just does a lot of things differently. I would assume that it uses some kind of dynamic recompilation scheme (there's no other way to reasonably emulate a 33MHz MIPS on a 200MHz SH-4), uses the PVR for rendering, and the DC's sound system for sound playback.
While it's true that the PlayStation and Dreamcast are more similar, and that should allow a number of shortcuts (like being able to offload rendering and sound playback to the hardware) which aren't possible for a SNES emulator, the PlayStation is also a hell of a lot faster than a SNES, and has many more pieces that would need to be emulated.
It's not like the N64 either. The N64 had a complex 3D system, which had something that's more-or-less equivalent to modern day vertex shaders (RSP microcode). However, Nintendo provided a standard set of libraries to interface with all of the hardware, and initially forbade developers from hitting the hardware directly. Developers were especially forbidden from messing around with the RSP microcode. Early N64 emulators didn't even attempt to emulate much beyond the CPU. Instead, they located Nintendo's standard library, and replaced it with native implementations of the same functionality.
On the PlayStation, Sony provided a development kit, but it was kind of useless, so almost everybody skipped it and hit the hardware directly. There's no standard library to intercept, so a PlayStation emulator has to emulate most of the system's hardware as well.