I think it's reasonable to assume that both your negative reaction to methylfolate and your symptoms, suggest you may have a methylation problem. The negative reaction you had to metafolin might be induced folate deficiency or the increased potassium demand. Your intake of methylfolate is just enough to fire up the methylation cycle and all of the sudden, you need more metafolin. So you get negative reactions (depression). It's a little counter-inuitive, but when it makes you feel bad, you may need more.
My wife has been 95% healed for over 2 years using Freddd's protocol stickied at the top of the page. The active b12 protocol. Please note that the jarrow mb12 is no longer considered effective by many people on here. If you can "handle" 800mcgs methylfolate and are already injecting methylcobalamin, Freddd's protocol might be an easy enough transition for you. The other protocol used by many is Rich's protocol, also stickied at the top.
I suggest reading up on the two protocols and picking one based on your desired aproach. The active b12 protocol (Freddds) takes a direct approach and one must "embrace" the symptoms (depresssion, any many others) in the beginning and use them as signs to titrate more supplements. You supply your body with the active forms of the supplements so no conversion is necessary. Some will say that this overdrives the methylation or causes detox. This is a huge difference of opionion on this message board. Having seen the active b12 protocol work miracles in a few short months, I am a little biased, but I believe the "detox" and "overmethylation" are often confused with induced "paradoxical" folate defeciency or potassium demand. This can be tested trating the "startup" symptoms away with mb12, adb12, potassium, and methylfolate.
Rich's protocol is a lower slower approach that makes the assumptions that your body will convert "inactive" forms of b12 and folate at the rate it needs them, and this slowly lifts the methylation block. Whatever you decide, stick with it for a few months. Work within the supplements listed in the protocols. People have a tendency to start customizing the protocols and adding outside supplements for many different reasons and it gets confusing and overwhelming. Keep it simple, work within the protocol.
As far as your injecting mb12 for Pernicious Anemia, the sublingual mb12 could take care of that. Check with your doctor, but most on here get better results with the b12 lozenges. The injectable forms can degrade with light and have other inherit problems. Don't look at treating methylation as another symptom. Look at it as the root cause of all your illness.
Final note, watch potassium demand and don't start without some on hand. My wife battled cramping and IBS symptoms and had a large potassium demand for months when she started the protocol. Some have theorized that its your body healing and using all the potassium. Whatever it is, if you don't have supplements on hand, it causes a lot of problems and can be lethal.
Its a lot to take in and very confusing, post any questions you have here and someone will usually answer them.