To your initial question, I updated my original 2.68 Simplex + to 2.77 and preferred the additional Park program, which was the 'new' Park 1. The original Park program became Park 2, and the Park 1 program had better break-points as well as (for me) in-the-field -performance.
I adjusted my Simplex + (had 3 by the time I was done) units so that all of he Disc. modes, Park, Field and Beach, were set at turn-on to only reject the first single Discrimination segment because that typically rejected most Iron Nails but let me still hear some of the more-conductive ferrous debris. Also, I would usually start up in the Beach mode because the 2-Tone function had a high-pitched Tone more like the Park 1 mode's High-tone. Otherwise I would opt for the field mode. Those 2-Tone modes, on my Simplex +, worked well for me in maybe ¾ of the sites I selected at the time. The other ¼ of my hunting time was more urban Coin Hunting with modern trash and that's when I opted for Park 1.
A Dealer friend had a Simplex + and updated to 2.78 when it was initially offered, but didn't like it and reinstalled 2.77 which he also preferred. I did sell my Simplex + outfit to some good friends and he loves that unit, especially for freshwater Beach Hunting. I don't need it but I do kind of miss it and wouldn't mind getting another Simplex + that has the 2.77 software. Wouldn't care for the 2.78.
Back to search coils. I used the 11" initially when the unit was released, but not a lot because I don't usually care for a larger-size coil. Then I got Nokta's SP-24 which is their 5x9½ DD and it felt so much better in weight and balance, and performance was quite impressive. Mounted it and never took it off! Best all-around coil, in my opinion, for the Simplex +.
As for 'after-market' coils, I initially thought about NEL because they said they were making coils for the Simplex + and that included the 5" Sharp DD. I talked with Dilek at the time and asked if they were going to make a small coil for the Simplex + and she said "No, probably not." I asked about the NEL coils and she said she didn't think they would make them because they hadn't given them the chip information. They did provide the info to Detech but only to make the 13" Ultimate coil for the Simplex. NEL wasn't provided the design 'chip info', or at least not at that time. So I don't know how their 5.5X9.5 coil would work with the Simplex + of any version. I know NEL got me a first release 5" DD Sharp coil for another new model entry back close to that time, of a different brand, and it worked .... sort of. The Audio responses were generally pretty good and clear, but sometimes the Tone ID would be off a little because the Visual VDI read-outs were several digits off from what all of the manufacturer's available coils produced. Also, with the 5" NEL coil the read-outs were not as 'tight' so you had a bit wider spread that were off from what the target typically registered. I wondered at the time if they needed certain 'chip info' from that manufacturer, and if they coils would not have been as good as those made by Nokta for their own Simplex +. Then Nokta bought out the SP24 5X9½ and I was satisfied with the Simplex + performance, although I do wish Nokta would make a small coil for it.
I haven't used a NEL coil on the Simplex + so I don't know how it performs or compares to the SP24.