Mitosis is not a problem. In mitosis, whatever is there just gets copied, the homologous chromosomes do not align with one another. Meiosis is where the problem arises, so a bit of background:
Consider 3 generations, call them G for grandparents (4), P for parents (2), C for child.
All human cells except sperm and eggs are diploid, with two non-identical copies (homologs) of each chromosome. Each P previously inherited two homologous copies of each chromosome, one from each G.
Meitoic recombination occurs in P’s sperm and egg cells. P’s two homologs align and a carefully orchestrated process of recombination occurs, where the DNA strands are cut and crossed over, resulting in new chromosomes each of which is a cut-and-spliced mosaic of the two G homologs present in P. Sperm and egg cells are haploid - they get only one copy of each recombined chromosome. The crossing-over is random, so each sperm & egg has a different mosaic mixture of the two G homologs. For meiotic recombination to occur, the karyotype must be identical. Thus, a horse P can generate viable sperm & eggs, because both G were horses with identical karyotype (64 chromosomes), so recombination can occur. Similarly, a donkey P can generate viable sperm and eggs, because both G were donkeys with identical karyotpe (62 chromosomes).
The two P’s mate, and upon fertilization the single copy of each chromosome in the sperm and the single copy of each chromosome in the egg just come together in the zygote. There is no alignment required at this stage, and not even any copying of DNA. So if one P is a horse with a karyotype of 64 chromosomes, and one P is a donkey with a karyotype of 62 chromosomes, that doesn’t present any problem per se at the level of the molecular machinery. C is a mule zygote with 64 horse chromosomes (each a mosaic of 2 G horse chromosomes) and 62 donkey chromosomes (each a mosaic of 2 G donkey chromosomes).
Mule zygote C’s cell can now proliferate to develop into a mature organism. The DNA copying in mitosis does not involve any alignment of homologs, just copying of what’s already there, so again the uneven number of chromosomes does not matter at the level of the molecular machinery per se.
But the problem arises in generating sperm or egg cells in the mule C, when meiotic recombination (as described above) is attempted. At this stage, the uneven numbers of chromosomes cannot all find a partner to match up and recombine with, and meiosis fails. It fails because the process of aligning and cutting and splicing homologous chromosomes involves carefully orchestrated and highly complex molecular machinery, with many careful checks and balances, and it generally won’t just “make do” - if things don’t look just right, it won’t complete successfully. Thus, mules cannot produce viable sperm or eggs.
I’m going to post this, then come back to answering your specific question.