I don't think that a PC adapter on your D750's hotshoe is going to work the way you envision, because the camera only allows you to either using an external flash unit on the hotshoe or the pop-up flash, not both at the same time. Once the pop-up is popped up, the flash hotshoe is disabled (and once the camera senses a speedlight on the hotshoe, the pop-up is disabled), so your adapter doesn't actually have any signal to relay.

To avoid having your finger over the sensor, you could use the head swivel capability, so that you can rotate the body in such a way that you're gripping it without covering the sensor window. Or you could get a cheap ringlight flash, and use that instead of the pop-up flash, so you can sandwich your PC adapter between it and the hotshoe, and be able to fire both flashes.

But your best bet is probably to get radio triggers and a second flash. There's a reason so many of us use them. And this will give you a good, basic off-camera lighting set up for portraits as well as macros. They have no line-of-sight requirements (I.e., no sensor-has-to-see-the-flash-burst). And getting a second external flash will free you from having to use the pop-up flash (which is limiting you to only on-axis light from very near the lens and lower power levels, anyway).

For example, you could get a Yongnuo YN-560-TX transmitter, a Yongnuo YN-560III, and a Yongnuo RF-603II or RF-605 transceiver unit here in the US for a grand total of around $150 (obviously, there are a lot of other choices out there for this type of gear at a variety of price points and reliability reputations). The transmitter would go on your D750's hotshoe, the 560-III has a receiver built-in, and you'd put the RF-603II on the foot of your SB-26. And while you'd still have to dial in the power on your SB-26 (which you're probably already used to doing), you'd then have two off-camera lights that can be fired over radio, without the range or line-of-sight issues that optical slaving, and you won't be limited to the length of a cable.