What sealant should I buy to secure cable in brick

Hopefully those holes are actually sloped upward the entire way, but I would not count on it.

Silicon would be OK, especially deep in. But there's a product called Mor-Flexx which is specifically made to look and work like mortar. A tube of this would work better than any mastic, as you can really squirt it far in. Plus you can touch up any other cracked masonry you might have around the house.

I might also solve this exact problem using rapidset mortar in a pastry bag (to get the mortar deep into the brick). But this can crack brick over the decades if the mortar is harder than the brick and thus expands differently (a problem in historic renovation, as older bricks are often softer than modern mortar).

That said, the Mor-Flexx is easy and will solve the problem completely. You can then install a weatherproof cable entry point and all future installers will take the lazy way out and use it (who wants to drill brick if you don't have to?).

Sorry, but I don't think you are allowed to do this. See Cat 5/6 in same conduit as power: is NM-B required, or is THHN acceptable? for more information.

So while there may or may not be any actual technical problem, there is a code problem. Would there be a problem running your Ethernet outside the conduit, just inches away? According to code, no problem at all (as far as I know). Would it cause any technical (signal quality) problems? My guess is probably not, especially for just a few feet.

But you can't run it inside the same conduit. You can (and this is a frequent suggestion when pouring concrete or digging a trench) run multiple conduits and have a small one for network/phone/etc. and a large one for power. But you can't just throw everything into one conduit.