Choices for recording during interview

A lav would be my best guess for this, but know very little about video still. I think the best you can do is place it well, and clean it up digitally afterwards. I don't see how changing recorders would have any effect, and a shotgun mic, altough perhaps useful either way, probably isn't gonna give you less ambient noise. The recorder choice could make it much easier to move around.

That said, I like the zoom and tascam handhelds for what they are, and have had good results and reliability with both. Using them for band practices and riff capture worked great.

For mobile video I've had my eyes on the sound devices new offerings. Particularly this one, the mix pre-6. https://www.sounddevices.com/products/recorders/mixpre-6

My buddy has one of the grey sound devices from the older flagship line and it sounded good on acoustic guitar in a room via AT3035 mic, and also mixes straight off the board from clubs.

Being battery powered is so crucial. Having power to charge those, equally as critical. I like the mix pre cuz it has using the dslr for camera in mind. Which is exactly what I had in mind before discovering it, and exactly what your doing already, i think.

Also some of the other scarlett interfaces are bus powered, so maybe that and a laptop would get the job done. My Scarlett solo has a footprint not much larger than a smartphone, and as portable as any interface I've seen.

I've had good luck over the years with the Sennheiser ENG series, pocket pack receiver straight into the camera. My ENG kit also came with a plug in module to make any dynamic (or self-powered condenser) mic wireless.

Would the customer be dead-set again using a small discrete headset / earset mic instead of a lav?

You and I know 'affordable' is a relative term:). You can easily find earset / headset mics anywhere from $20 - $2000.

Of course you could always go with a Sennheiser. Every company has their own specific connector, pinout, and resistance values etc. So any old mic with an 1/8 TRS may not necessarily work, or have poor performance due to mismatched specs with your Sennheiser pack. Their discrete models aren't cheap, their cheapest one would make your client look like an aerobics instructor, which probably isn't going to work. Generally, the more directional they are the less discrete they are. Their HSP 4 is a pretty good compromise. I your case you will have to balance the need for directionality in the noisy work environment vs. the tiny (almost invisible) omni mic that will pick up everything.

Countryman mics are commonly used for that sort of thing. The H6 is a good quality headset. The Isomax is much more obtrusive, but very directional and less prone to feedback. They come in a variety of shapes, colors, and sizes, and even mic patterns. The sound quality or their earset mics is also quite good, once you find one that you like the fit. When I'm doing an install or rental where the mic will be passed from one person to the next, I try to go with a headset, rather than a earset. The fit is more universal. The earsets are better suited for when you can let one primary user bend and fit the earset just for them. The Countryman mics let you choose the cable you need to connect to any brand transmitter pack.

I've never had an opportunity to try any of the DPA headset mics, but I think it's safe to assume they are top-notch as well. I wish I had some first-hand experience with DPA to share, but I don't.

If I see less expensive headset/earset mics by any half-reputable brand, I'll often buy one to demo, hoping to find better values for my customers. Most of them turn out to be absolutely unusable garbage and I just give them to somebody. So far, the least expensive brand I've found that still sounds pretty good is EliteCore / OSP. They also have single-ear earsets and headphone versions available. At a fraction of the cost you have to ask yourself, "is the difference in sound quality something I can justify spending that much more money on for another brand in THIS application?" They also use a field-replaceable cable tailored to whatever brand transmitter you're using. These are a good fit for me outfitting school theater productions, the kids are hard on the equipment, so if they break the cable between the pack and headset, they actually give you a spare cable along with the mic. If the kids destroy the whole thing, it's not the end of the world, because they weren't super expensive to begin with. I've sold scores of these and have been very satisfied with the cost vs. performance ratio.

Unfortunately, my favorites over the years in terms of sound quality (Audix HT5, and AKG HC577) wouldn't be compatible with your Sennheiser gear.