We have a family cabin in Shaver Lake and we've spent a lot of days at China Peak near Huntington Lake.
The easiest route between Shaver and Huntington Lake is via Highway 168. There's a lot of elevation gain between the two lakes and it would be a strenuous ride with 2,500+ ft of elevation gain, even for an ebike.
An alternate route between Shaver and Huntington Lake is via Huntington Lake Rd through Big Creek. This is a windy road that descends down to Big Creek, then back up to Huntington Lake, also with a large elevation gain for a bike.
There are a ton of fire roads between the two lakes, but a lot of them end up meeting back with other main fire roads. Southern California Edison (power company) owns a lot of the land up there. There are a lot of utility trucks and employees on the highway and main fire roads this time of year around Shaver Lake. The highway is somewhat up on a ridge between the two lakes. If you get off course on the north/west side of the highway, it's a steep descent down into the town of Big Creek. If you get off course to the south/east side of the highway, you would likely end up back at Shaver Lake.
Shaver Lake has a volunteer fire department, a sheriff's substation and a seasonal ambulance substation. There are a few grocery stores, a hardware/sporting goods store, restaurants, and a gas station that are open year-round if she needed something.
Cell reception is decent throughout the area for Verizon and it's okay for AT&T.
Mono Hot Springs is another large trek from Huntington Lake via Kaiser Pass Rd. It's an additional 2,500+ ft of elevation gain, another strenuous ride on a bike. There are a few dry campgrounds off of Kaiser Pass Rd before making it to Mono Hot Springs.
Tamarack is a small neighborhood of cabins about halfway between Shaver and Huntington with fire roads that take you to the south side of China Peak, to lakes like Strawberry Lake and Red Lake. South of this area is "Dinkey Lakes". These areas are popular for rural backpacking and rock crawling, but still pretty quiet this time of year. But this is all the opposite direction of Mono Hot Springs.
I think I would rule out sightings in Big Creek, Tamarack and Dinkey Creek first. Then focus on Kaiser Pass Rd. If they are sure she made it to Huntington Lake, then I would focus only on Kaiser Pass Rd and the surrounding areas towards Mono Hot Springs.
Mammoth Pool, Edison and Florence would also be areas you would want to rule out if she made it to Huntington Lake.