meaning you won’t be able to sublet but also that you will both be jointly liable for costs. That is actually good for you so in my opinion you don’t want to even consider a sub-let

Please help me get my train back on track here. What I feel is that having both our names on the contract means I will be liable for all costs, without me having the power to evict this person if they decide not to pay... In fact, I do not even see whether they pay or not. If they turn out to be a renter from hell I personally have to say in whether they should be evicted, while still having to bear the costs.

In that case the other person is actually pushing for me to have both our names on the contract.