"SF Twin Reverb, ultralinear 135w beast"
I will take this as a "given" for my answers to the rest of your questions.
"My amp man says doghouse caps orig (silver mallory) & read '78.. but chassis tells me '75."
What do the transformers and pot codes say? This will confirm or deny the caps. Also - Fender bought caps by the bazillions. Sure, some could have sat around for 3 years in a bin somewhere before being used. Same for pots and trannies and any other parts.
WHAT on the chassis dates your amp as a 75? Serial numbers cannot be used this late in the game, and any tube charts in the cab were never to be trusted entirely. And even if there is a stamp inside saying "75" or whatever, that's likely to be totally random. As far as I know, Fender never directly dated a chassis.
"The spkrs are 50w jensen RI C12N's (BF mod rewired tubes for pentode or s'thing or other, ie 1/2 power 65w anyway, hence these spkrs put in). Any probs having 'only' 100w total? I'll never be diming the thing."
If you are indeed restoring it to 135W stock, then you may well likely go o w your 100W worth of speakers, domed or not.
A Fender or Marshall 100W can put out 100W long before you can perceive it. And depending on how efficient those speakers are, you may never even HEAR 50W let alone a full 135.
Basically, if I take it on faith that you are truly restoring a 135W Twin Reverb to "stock," then, hell no it's not safe to use with 100W of speakers.
"It has a repro BF faceplate (vg quality, I have a DR one with the same pre cbs spiel on).. so I can't tell whether it ever had an MV, as this obvioulsy isn't poss with no hole on the BF faceplate. So, I really need to ID the age of amp > then determine if it falls in the 'defo had an MV' range of dates."
Fender Master Volumes were issued in 1974. So EVERY Twin Reverb from 1974 til 1981 came with a Master Volume. Look behind the faceplate, bet you'll see unused hole where the Master Volume used to be.
"Lastly I see this interesting (similar SF twin) YTube clip, saying 'reverb undone/ rca patchcord from in to out.. gives OD boost' here anyway: safe/ general thoughts?"
This used to be an "effect" called an "Ice Cube." It was a 470k resistor between two RCA jacks. It's the same thing Fender used to implement their notorious/infamous "Pull Boost," switching the reverb tank out for that resistor. General consensus is that it sounds like butt-ass-crap. I tried it once, good if you're into Ted Nugent and don't mind 10x the noise and hiss.