I have connected in SAL many times on TA. OK airport and as prior person said Lounge not so great. SAL can be rather hectic if you are arriving/departing during on of TA's banks of flights to North/South America.
I fly into and out of MGA all the time. I don't believe you can transfer airside and have to go thru customs. Presidente Ortega extorts $10.00 from you for his Ice Rink. Airport is nice enough with a Pay for Lounge there and lots of Duty Free shopping. Tona beer and Flor de Cana Rum are great buys.