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Depending on how the citronella plant is growing, it may be possible to divide it. If you are growing Pelargonium 'citrosum', which is the geranium that smells like citronella, you can look to see if you have multiple plants. This plant grows in rosettes, and each plant is its own rosette. You can separate these rosettes and repot them each in their own container.

If your are growing Cymbopogon, also called lemon grass or citronella grass, this can just be divided as you like.

Both plants respond well to pruning as well. You just cut back the plant by about a 1/3, and it does not matter much where the cuts are made.

Roses are a different matter though. This article will help you with pruning those:
https://www.gardeningknowhow.com/ornamental/flowers/roses/pruning-roses.htm

This time of year, all plants that are overwintered to simply survive look a little pathetic. Repotting won't help right at this moment to improve how it looks. Try to get it more light and humidity. Water it only when the soil feels dry to the touch at top. Then, acclimate it (https://www.gardeningknowhow.com/houseplants/hpgen/move-houseplant-outside.htm) to move back outside as soon as the weather allows. Even if it looks really ragged by the time it moves back outside, it will bounce back quickly and look beautiful in no time. In short, wait to repot as it will not help now.

This is a very common growth pattern and can easily be remedied by cutting back the plant.
Here is a link to the care of scented geranium.

https://www.gardeningknowhow.com/edible/herbs/scented-geranium/scented-geranium-care.htm

It sounds like a fungus and can be treated with a fungicide.
You can cut back the effected areas of your plant and remove from the garden.
Wash tools you use before using again in your garden.

Here are a few links.

https://www.gardeningknowhow.com/edible/vegetables/cabbage/cruciferous-leaf-spot.htm
https://www.gardeningknowhow.com/edible/herbs/scented-geranium/scented-geranium-care.htm
https://www.gardeningknowhow.com/edible/herbs/scented-geranium/mosquito-citronella-geranium-plant.htm

It appears to be either over watered or under watered. I am going to guess that the plant is in a container outside and that it has proper drainage, so it is likely under watered. Soak the whole container in a tub of water for 20 minutes, then make sure you are watering at least once a day, 2X a day in temps over 85F.

Yes you can trim it back.

Cut it back to a group of leaves as far down as you would like.
You can propagate the cuttings.

https://www.gardeningknowhow.com/edible/herbs/scented-geranium/scented-geranium-care.htm
https://www.gardeningknowhow.com/edible/herbs/scented-geranium/mosquito-citronella-geranium-plant.htm