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I have only done a few HDR images. But I believe the recommendation is to avoid making adjustments to the individual images prior to creating the HDR image. That's the way I have always done it. I have just used the original raw files and allowed either Lightroom or Camera Raw to create the DNG HDR file, and then made the adjustments in that file. But as long as you are experimenting you might as well try every angle and decide for yourself what works best for you
Adjustments you make prior to merging are useless, because they will be ignored. You may have another problem however. You write that you have three exposures, the last two being "one exposed for the room with available light, and one exposed for available light but strobed to fill in the shadows". That means that these two images are not bracketed, but have the same base exposure. There is a good chance that the merge will fail, because Lightroom will detect that and refuse to merge them. You may have to merge only two: the one for exterior and the one exposed for available light but strobed to fill in the shadows.
I've gotten very good results using what you listed as 1) Just use Photo Merge > HDR, then adjust the DNG file. LR Merge to HDR creates a 16 bit Floating Point DNG file with Develop settings copied from the most selected image in your bracket file set. There's no need to apply ANY settings to the individual raw bracketed image files.
You left out one other HDR workflow option: 5) Use Lightroom's Photo> Edit In> Merge to HDR Pro in Photoshop. With this workflow the current Develop settings are applied to each of the bracketed image files inside PS. In general your LR Default Develop settings should be fine so no need to make any adjustments to the raw files. Choose Mode> 32 Bit, which will create a 32 bit HDR file that is editable in LR.
I agree with JohanEl54 concerning the bracketed exposure settings. In general creating three image files with -2, 0, +2 EV exposure using your camera's bracketing mode is all that's required. If using fill-flash it should be set to manual mode, kept on for all three exposures, and use Aperture priority to bracket the exposure. Don't forget to allow for flash recycle time between bracketed shots.