Fresh install 5.0.1 topic

I am getting a few issues with 5.0.1 such as lost data connection and the dialer dropping calls. I have had this device since 4.4 so many OTA's so would like to do a complete fresh install. Should I download the factory image and use the flash-all command in adb? Will this erase all data, which is what I want, replace all radios etc.

[Q] Which Firmware W/ Fresh SlimKat 9 Stable Install topic

Background: I attempted to update from SlimKat 7.0 stable to 9.0 stable and discovered my apn's were missing and when I try to add them manually it fails to save. I have also found others with this same issue but none have an easy fix. I'm on UCDLK3 firmware, after restoring to previous state using nandroid backup apn's are present. If I try to flash any ROM (even other than Slim) my apn's are missing, even a dirty flash of 7.0 over 7.0. I know I can fix this by flashing stock UCDLK3 and root/twrp/flash slim 9 and everything will likely work fine. I plan to do just that but want to make sure I'm doing things as smart and clean as possible. There is a lot of info out there and I'm finding it hard to sift through old stuff and find the current state of things.

Question: Is there an actual advantage if I upgrade the firmware from UCDLK3 to UCUEMJB while still planning to use SlimKat 9 Stable with BMS kernel and TWRP recovery? Are there any disadvantages? Either way what is the shortest path to get back to stock (either LK3 or MJB) and flash SlimKat?

Thanks