Why would you need those "missing" blocks?   The way it works is that when you apply a particular set of timings for a particular clock it means that this set of timings is used at that clock and above. So when you've got lines in your bios for 1500, 1750, 2000 and 2250 (the way it is for rx 560 cards with hynix vram, for example), you can use one set of timings for any clock between 1500 and 1749, then another set for clocks in between 1750 and 1999, then yet another set for anything between 2000 and 2250, and finally another set for anything above 2250. If your last line in BIOS is the 1500 MHz strap, then all it means is that when you put a particular set of timings in there, those timings are applied at 1500 MHz and any clock above that. Having the max strap in the bios at only 1500 MHz makes no difference whatsoever, for mining purposes you usually just pick one set of timings and then try to find the highest memclock the card can work at. Just put the timings you want to use on that 1500 line and as long as your memory clock is 1500 MHz or anything above that — those timings will be used.

Straps — sure, there's a ton of them on the internet, just gotta try what's working for you (will depend on how much you can overclock your vram). At the moment I'm using
777000000000000022339D00CE515A3D9055111234CC440900400600740114206A8900A00200312 01E0F292F94273116 for Hynix and
777000000000000022AA1C00315A5436A0550F15B68C1506004082007C041420CA8980A9020004C 01712262B612B3715 for Micron.
There are probably better straps out there, these are just the best I've managed to find so far.

As for unlocking those extra cores, this worked for me: https://www.reddit.com/r/Amd/comments/6gah41/how_to_unlock_your_xfx_rx_460_single_slot_4g_by/
All it takes apparently is to change "70" to "6F" in the BIOS. My 560 14CU cards all got unlocked with this simple edit.