The interim solution provides the most compact solution, taking no more space than that necessary to show the selected value. The split menu you use to allow access to the most frequent items has been shown to work well (Sears and Shneiderman, 1993), and it’s been around for a while, so it may be familiar to users.
Your possible alternative reduces the clicks for the most common items from two to one, which may very well be worth consuming the added space. If you use text labels instead of images, you’ll probably be able to save even more space (or fit more high-frequency items).
The largest potential problem is ambiguity about the relation between the combo box and the frequent-item controls. Usually, different controls implies separate fields. Do the users choose from one or the other, or from both? How do they know? Where do users look to see what they selected? Focus is divided in this design.
Some design details may mitigate the issue. The combo box probably should automatically set to match a selected frequent-item controls. Likewise, the frequent-item controls should set to match any choice made with the combo box if it’s one of the high-frequency items.
A possible issue with your particular design is that frequent-item controls may appear ambiguous to the user. I’m guessing they are toggle buttons, but users could mistake them for command buttons that might take them somewhere they don’t want to go. Worse, with no borders around the controls, the user might mistake them for mere illustration –just examples of what you mean by “appliance.” A one-of-many selection is usually represented with radio (option) buttons. That would mean the odd situation of sometimes no radio button being selected (because the user chose from the combo box), but they might adapt to it.
A Third Possibility: List Box with Split Menu
I think you can avoid such complications by using a list box with a split menu rather than a combo box. It’ll take the same amount of space as your alternative (show all high-frequency items, plus the top of the full list without scrolling), and still provide one-click selection of the most frequent items. However, now it’s a single control so it’s a single point of focus for the user. It’s more familiar than a combo-box-with-radio-buttons design.
Fourth: Search/Filter/Sort Enhancement
You mention items may have unexpected spellings that may make them hard to find in a long list (probably true with scrolling or type-and-filter). You say there are no categories, but are there other attributes that users may use to filter or sort items? Cost? Geographic position? Can you use a search algorithm that is more tolerant of spelling errors than type-and-filter?
If so, you may want to include, in addition to the split-menu combo box or list box, a “More…” button that takes the user to a page or dialog box for searching, sorting, and filtering in various ways on various attributes.