I would also certainly encourage you to use a good poultry vitamin supplement as it will support the flock's immune system. One of the side symptom's of Marek's is that it suppresses the immune system, so anything you can do to support it is beneficial.
If a bachelor pad is not an option, then you need to start culling some of those cockerels in the next few weeks before hormones kick in. At 10 weeks there will not be much meat on them, so perhaps keep the better behaved ones a few weeks longer. There is no apparent health risk from eating the meat of Marek's infected birds although one of the many symptoms is muscle wastage, so there may be even less meat on them as a result of that. Often Marek's birds are smaller even before they exhibit any symptoms, so I would select the largest two cockerels with the best temperament and whittle it down to one when their true personality starts to show. The problem with keeping more than one adolescent cockerel in a flock where there is no adult rooster is that they will competitively mate the females and often the lowest pullet in the pecking order will get mated repeatedly by both males one after the other. It can make their lives a total misery as well as cause a risk of injury, so be observant and have a cage available to put one or both in a sin bin if necessary.
As regards bringing in other chicks, it is a risk even if they are vaccinated. Egyptian Fayoumis are supposed to be Marek's resistant and might be worth a shot. Silkies, seramas and Legbars seem to be pretty susceptible, so avoid those. Adult hens might be a better bet than chicks because they are past the most vulnerable age for the virus to manifest, but they could bring with them other viruses that might cause any birds in your flock with compromised immune systems to succumb, so a rigorous quarantine would be necessary and careful integration so as to avoid stress.
There are no right or wrong answers, it's just a question of observing closely and doing your homework and trying to weigh up all the risks and make an informed decision based on your own set up, goals and flock dynamics. Do you have chickens other than these young Icelandics? I had a mixture of breeds and ages when it broke out in my flock (including a mature cock bird), which was probably a benefit as most were past the most vulnerable age and I was able to breed from the healthy ones. Free ranging them also helped, both from the point of view of keeping them happier and also less exposure to the infected dander dust because they were not confined together.....so there are a lot of factors to take into consideration.
From what I have read, the vaccination which is available to buy for back yard flocks is the least effective of the vaccines that have been developed and may in fact be responsible for the more virulent strains of Marek's currently circulating in the states, because it allows the virus to mutate.... Marek's outbreaks seem to be milder here in the UK where most backyard flocks are not vaccinated.... each loss is still devastating of course but at least it is just the odd one or two, rather than the majority of a hatch as some people in the states report. It may therefore be short sighted to vaccinate as it should protect the chicks from the worst of the Marek's symptoms, which cause fatalities providing that strict bio security is observed during the first 2-3 weeks after vaccination, but in the long run it may mean that the vaccine becomes less effective and the virus more aggressive.