Could this Francobelgian comic book woman really have these 4 diseases at the same time?
Erythema multiforme (minor) can and does occur in a lot of people; while it is usually self-limited, it can recur, especially when the trigger is an unsuspected food.[1] How common is it? It is very common.

Necrobiosis Lipoidica is not uncommon in diabetics, less common but still found in non-diabetics. It can occur at any age, including the eighth decade. It also shows a sex predilection, being 3 times more common in women than in men.[2] How common is it? It is not rare.

Phlegmasia Alba and Cerulea Dolens[3] is serious and associated with deep vein thrombosis, which is an acute event. It is not uncommon, but it's not chronic. (More than 600,000 cases of venous thromboembolism are estimated to occur each year in the United States, all of which can lead to Phlegmasia Alba and Cerulea Dolens. How common is it? Luckily, not very. If the author is referring to garden-variety thrombophlebitis, though, that's a chronic problem, and not uncommon in the elderly. Thrombophlebitis is chronic and common enough.

I'm not going to address Metropathia Haemorrhagica, because, as you have noted, it's found in menstruating women.

Diseases generally don't really interfere with each other; some diseases can't co-exist (you can't have hyperthyroidism and hypothyroidism at the same time, for example, or have an active pituitary gland tumor and empty sella syndrome at the same time), but diseases don't interfere with each other exactly. Some diseases have common underlying foundations, so can occur co-morbidly, for example collagen vascular diseases can manifest with skin, joint, and lung findings, which might not be diagnosed as several manifestations of one disorder if it's not obvious. A better example might be diabetes, where similar underlying pathologies cause, say, heart disease, skin disease, eye-problems and kidney disease together.

Whatever, the likelihood of a little old lady having all four of your diseases are nil. The first three all the time (assuming not garden variety thrombophlebitis) - nil. Once, not impossible once, but not chronically. Of the first two - not far fetched. The first - common as dirt (comes and goes, though.)

I hope this helped. 

[1] Erythema Multiforme on Medscape
[2] Necrobiosis Lipoidica on Medscape
[3] Phlegmasia Alba and Cerulea Dolens on Medscape