I differ from the opinion of @CodeGnome. Processes and plans cannot be one-sided - they need to be agreed on by all parties to the project or any system. In your case, the client is clearly not in agreement with you - for reasons that you have not spelt our or perhaps you are unaware of. Neither is your senior management willing to work with you to solve your problem. Why is that?
You cannot have a "plan to release every 3 weeks" when the customer is delaying the UAT by 4. It would appear the client is not participating in prioritizing work for the next release, nor in reconciling change requests with original scope of the project (altho' your sentence "but these are after some minor change requests to the initial work conflicting" is not very clear.)
In my opinion, you have to either find someone who has the respect and/ or authority to conduct this discussion or take it upon yourself - it might very well be you since you are leading the team - to have an open and sincere discussion about the challenges you and your team are facing, and by that token, so is the customer and so also the management. After all, there is a financial cost of delay which the customer is incurring! By all means, also be prepared to offer solutions - such as getting your team to pitch in to write UAT cases (there must be a reason why they are delaying UAT!) or anything else that helps resolve things.
You all need to get to the root cause(s) of the situation, work out a process that everyone agrees to, and the objectives - both functional and financial - of the project. Only then can you see success moving forward.
In my (far too many) years of experience, I have gone thru many such experiences (difficult customers, reluctant management) - and what has ultimately worked is for sane men and women to get together, get some perspective and resolve things. Unfortunately, almost always, it takes far too long - and the people who suffer the most is the team working on the project.
Hope this helps - and you are able to have that conversation with the customer. Once you have agreement on the process, by all means, 'stop the line' the next time this situation develops.