Tigase BOSH client scalability

Tigase normally has no problems with handling 10k online users on a single machine.

ad. 1. This depends on a few factors - hardware you use, operating system, load level on the server, traffic type and whether you can use keep-alive connections. Normally 10k online users is a low number and it should be handled by Tigase on a decent, single machine without any problems.
ad. 2. Bosh connections in non keep-alive mode are a bit specific and they put a very different load on the server. So in large deployments many connections managers may indeed improve scalability. I don't think however this would be necessary for installation with 10k online users. As I mentioned above however this also depends on many factors. It is hard to say without any more details or tests.
ad. 3. I don't know PunJab so I can not tell too much about it. Tigase 'built-in' Bosh component can also be deployed externally and you can connect many Tigase Bosh components to a single Tigase server. This might solve the problem with scalability in some cases.
ad. 4. Yes, it may cause problems. It depends on the load-balancer. First of all it may be impossible to use keep-alive Bosh connections which increases load on the Bosh component. Secondly, if you have many Bosh components it is important that once the user authenticates on one Bosh component all further calls during the session must end up on the same Bosh component otherwise it will not work. If your load-balancer allows for this then you should be ok.

Instead of using multiple Bosh connection managers you may also consider to deploy Tigase in a cluster mode. It would not only improve scalability for your installation but also improve availability of your system.