I have the same malabsortion issue and had very painful muscle-cramps, which even up to 2.4 g per day of oral elemental magnesium only alleviated somewhat. RBC Mg isn't available to me, but found a very cheap whole blood magnesium test, which reflects the severity of the deficiency very well. Normal range is 34 - 36 mg/dl. My results:
2012: 31,8
2015: 29,9 (a further drop, despite having oral intake above 2 g/d)
2018: 32,8
Only End of 2017 I found a GP who gave me inexpensive Mg-Sulfate IVs (
https://www.patienteninfo-service.de/a-z-liste/m/magnesium-verlaR-iv-50-infusionsloesungskonzentrat/). After the 6th IV about April last year the cramps completely ceased, after the 10th IV I had the last Mg whole blood test (all the while condinuing my high oral Mg intake). Therefore with the 3 mg/dl rise it seems I'll need an other 10 IVs in total. Though somewhere I read 8 IVs usually would be enough to correct a severe deficiency, this doesn't seem to be the case with me at all.
In fact, didn't experienced any real PEM since getting Mg-sulfate IVs. Though that is heavily confounded by very strict pacing, having my only root-canal extracted, and taking LDN, to mention a few.
About injection I heard only, that they are very painful (while IVs only give a pervasive calm). Other possible options would be taking Mg-sulfate via nebulizer, enemas, or transtermally (probably most effective with DMSO).