Are there visa agents in Mukdahan who can assist with exiting and re-entering Thailand?

Yes. There are 'tour guides' at the border that can help with the process but it's very easy to do yourself.

Drive there park in the secure car park just before the bridge

Go to Immigration exit Thailand

Jump on the bus takes you across the bridge

Get your Loas visa on the other side enter Loas grab a taxi to Thai Embassy need to be there in morning 9am perfect ( across from the Embassy is Tony a road side office a Loas guy helps fill all the forms out for you photocopys ect ect)

If you need any help

Put all paperwork in

Then pick your passport up the following afternoon

All ways found it busiest on a Monday I go Tuesday or Wednesdays normally

Hope that helps