Transaction Replication as DR solution
The Transactional replication is not a "DR Solution" you may face a lot of issues with this, (Not all tables replicated, Different data in each DB) even that it worked in the past, There's no warranty it will work in the present. The suggestion will be using some supported tool like log shipping or always on. 

Even with that if you decided to go with this, as @Queue Mann said, you will need to point everything to the new server, There's no automated option on this, (but there IS in Always on) 
For your questions: A) There's no "official" fail over, you just use the replicated DB and point all your applications to the DB. 

B) In the case of fail over you will need to set up everything again (In Always on , you won't as long it was done in Synchronous mode). 

To be honest I don't know why keep using replication instead of the official DR solutions, you will save a lot of configuration (and troubles) going in the right way.