Heya,
I get more stabilization from a monopod than from IS. Effective use of a stable platform is going to give you huge advantages in slow shutter speeds. It's not as stable as a tripod + gimbal, but a monopod really helps you to stabilize the vertical plane. You can wedge the foot of the monopod into the inside of one of your feet and press down (down into the vertical plane) to tighten up how stable the system is in that vertical plane. Then you're free to pan side to side or simply take stills at very slow shutter speeds even with very long lenses. You can get way more than half a stop of stabilization using a monopod, if you use it effectively. But you have to use the lens differently, if you use it like a handheld lens, you'll get the opposite results. A monopod can be more restricting than handholding, which should be obvious but if you need stabilization or to take the weight off your arms, that's what it's for.
I get as low as 1/20s with a 600mm on my monopod, consistently, when I need to (more proof of concept than useful, I don't need 1/20s for what I do with that long of a lens, which is typically birds, but I shoot in dark canopies so I often am in very low light).
Long lens technique still applies, so you can rest an arm over the lens hood, or pull the lens hood down with a punched finger, to help stabilize vibration while the monopod stabilizes the vertical plane, leaving the horizontal plane free to move.
You can get a head for your monopod that allows an arca clamp as well as tilt feature so that you can come on/off of the monopod fast, and also, the tilt head is nice to be able to get a stabilized angle that isn't perpendicular to the monopod itself if needed.
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1. I either pinch my lens hood from below and pull it down (kills vibration); or rest my arm/hand over the lens hood (also to kill vibration). On shorter lenses, I might rest on the body and lens hood.
2. Nope, that's what everyone does. Just make sure you mind the connection of the collar to the monopod, if it's a screw on method, versus using a head with a clamp.
3. I use third party collars often, if it fits, it fits.
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I use a Desmond DMH-2 (this is a tilt head with an arca clamp, maybe $50?), which is a very stable and heavy duty tilt head for a monopod, with an oversized knob. It holds my camera and 600mm with plenty of room to spare. I sling it over the shoulder and it holds very firm, I don't worry at all using this thing. Best addition to my monopod. I use a Benro A38F monopod, nothing fancy, inexpensive. Between the monopod and the head and the lens foot, this gets tall enough to go over my head, which is useful for when tilting up if I want. I leave this tilt head permanently attached to my monopod. It has arca clamp on top, so I just attach to it any lens I want, and I use arca style plates on everything, so it's all universal.

IMAGE LINK: https://flic.kr/p/JQ5dsB
IMG_9886
by
Martin Wise
, on Flickr
IMAGE LINK: https://flic.kr/p/JPSLdf
IMG_9890
by
Martin Wise
, on Flickr
Here's some examples of slow shutter use on a monopod in low light:
1/40s
IMAGE LINK: https://flic.kr/p/JexMpC
IMG_3647
by
Martin Wise
, on Flickr
1/50s
IMAGE LINK: https://flic.kr/p/FLVsnh
IMG_2574
by
Martin Wise
, on Flickr
1/60s
IMAGE LINK: https://flic.kr/p/Kexqso
IMG_2176
by
Martin Wise
, on Flickr
1/50s
IMAGE LINK: https://flic.kr/p/Key83m
IMG_2093
by
Martin Wise
, on Flickr
1/100s
IMAGE LINK: https://flic.kr/p/Kvg6cN
IMG_2082
by
Martin Wise
, on Flickr
Very best,