First, patience please, this is Renfe, the most baroque of the large train operators in Europe ...
Wise move to Thursday if via Madrid (Friday and Sunday are expensive or Tren Completo), though it's 74.70 Basico on the 0902 + 1600. 2h at Chamartin might be boring. It's probably the safest as it's a single ticket and gets you there much earlier. Alsa is about the same price and far less comfortable.
It is actually possible on Thursday to use the via Léon connection you saw on Friday, 1158 from San Sebastian arriving Vigo 2335. On alternate days the Barcelona-Coruna/Vigo Alvia splits at Ourense(*) and you can buy a through ticket. On the other days the there is a MD Ourense-Vigo in the same timings. You can buy a through Vitoria-Vigo any day, but only San Sebastian on the other days. Eg Vitoria-Vigo is 44.80 Basico + 6.15 San Sebastian-Vitoria on 21/03 at present. Ourense-Vigo is in the dark so if cost doesn't matter then Madrid is nicer times.
(*) Edit - of course that split is currently virtual as both parts start at Ourense anyway. Also where you see a MD fare it could be on an Alvia service. It's Renfe, it doesn't have to make sense.
You're aware of Urzaiz (where you'll arrive from Madrid) up the hill versus Guixar (for Porto as well as direct to Ourense going back) down by the docks?
Eastbound, 0720 from Vigo is up the river Minho in daylight so beautiful scenery, very bendy, very slow, pick a seat on the right.
Rail replacement at Ourense I don't know about. I'd expect it to be immediately outside the entrance where the taxis are (the normal bus station is down to the right but I wouldn't go there unless told to) - I'd expect signing and just saying Monforte or Léon to people should work. Westbound, Monforte is very quiet so it will be obvious what is happening.
By the way, for spending all day on the train I'd pick Elige Confort. In any case, Elige to pay to choose your own seat - quick rule on Renfe is if the door is in the centre of the coach don't pick a seat at the end unless you enjoy a see-saw (26m coaches on mediocre track = nasty throwover).
Edit - a good reason for doing this now is that Adif have put out to tender work to convert the lines out of Guixar including down to Tui and the border to 25Kv AC. That includes an 8 month, supposedly, total closure to increase clearances in a tunnel. When that starts Vigo-Valenca and Vigo-Ourense direct will be bustituted. When complete, including ETCS at some point, more kit will be able to operate Porto-Vigo. Given Adif's record, eg Salamanca-Vilar Formoso is still shut years after work started, there might be no trains across the border until 2030 or so.