Quite some questions! Hahaha. I’ll try to do my best (excuse the scarcity of links, I’m writing from my FP2).
Hardware and mobile reception
The hardware is not edge, but is quite good. The overly-bloated Web is fine here with Firefox Quantum. I’ve using 70 installed apps with no problem, but with 2Gb of RAM and being a quad-core processor, you won’t have much problems with +100 apps, probably, but other people could help you more in this regard.
Mobile signal reception has been good for me in Spain (4g), but speed clearly depends on the network infrastructure. Note that the second SIM is limited to 2g. Some users have had problems with signal reception, search over the forum for testimonies and fixes.
Upgradability and aging
Noone knows if a core module will eventually be available, but IMO that would be more like a retrocompatible new model, probably (different core is just a different phone altogether in regard of kernel, drivers and OS. How will a user migrate from one to other?).
Fairphone just announced Android 7 will be available officialy this summer (for Fairphone OS and Fairphone Open), that together with monthly security updates will ensure a life-expectancy of 5 years (2+ from today). Said so, you have alternative ROMs available (more on this below).
Fairphone OS and Fairphone Open are the only OSes officialy available. Both have monthly security updates (this is important and rare in the industry!).
Fairphone Open doesn’t include GMS nor any other closed software apart from SoC blobs (drivers, you know…). FP Open has embedded the F-Droid Privileged Extension, which made F-Droid (after the user installs it) act as the main app store without security caveats. It is pre-rooted, so you won’t need to tinker a lot with random, unstrusted tools from the internetz. See more info about FP Open in https://code.fairphone.com or by the #software:fp-open category.
Apart from that, the Lineage OS Team officially supports Lineage OS for the FP2, with dual-SIM support and the expected quality. There are a lot of FP2 users here in the forum using it (myself included), and it has a category: #software:alt-lineageos.
You seem interested in ditching Google. GREAT. There is a lot of info here in the forum. Look at the #livingwogoogle topics. There’s an special build of Lineage OS called Lineage OS for microG which focus on integrating Lineage OS with the microG suite —an open source implementation of GMS to improve Android apps compatibility while giving you the control of your data & privacy.
The UBports project is maintaining a version of Ubuntu Touch/Mobile for the Fairphone 2. Updates of this are dependent of that community activity (not Fairphone). AFAIK, they are pretty active. Look at the #software:alternative-oses category.
The Sailfish OS community also has a port of Sailfish OS for the FP2. Again, updates are dependent of the community (not Fairphone), but they were the first project supporting Fairphone 2 and recently they have had a firmware update, so they are up and running. Look around the #software:alternative-oses category, too.