No, Apple's not done ARM SoC design "in a sense", they've done a from scratch ARM design entirely on their own starting with the A6. it is NOT an ARM design in any way. I thought this was common knowledge, surely there aren't still people who believe Apple is using an ARM core, or somehow relying on Samsung (Samsung hasn't even been fabbing their last few generations of SoC)
It is true designing a cellular baseband is not simple - Intel has had trouble but they've outsourced the whole project to a team of 1000 engineers in India, which shows the low importance they've placed on it. They don't design their CPUs in India...
Nortel's IP may not be useful for design, it would be useful however for patent defense - i.e. Apple infringes on patents X, Y, Z of Qualcomm or whoever, but they have patents A and B from Nortel for cross licensing. Qualcomm is the king of CDMA patents, but they are just another player when it comes to LTE, so once Apple decides to dump CDMA (probably not this year, but almost certainly next year since Verizon is dropping it as of Dec. 31 2019) trying to do their own baseband becomes much more feasible patent wise.
Apple will need to bring in some expertise, like they did with buying two CPU design firms to obtain the engineers necessary to design their own ARM core. They've hired some people, but probably not enough to roll their own. But they didn't go directly to fully custom ARM cores or fully custom GPUs. They started by licensing and modifying existing stuff, so that's probably their most likely path on the cellular side assuming they see value in developing their own.