Yes that's the stuff go for the lighter 450g grade
Wire brush the area clean first ,then spread acropol over the area you need repairing. Cut a patch and lay it over the area you have painted and then dab & paint over until the fibre glass has gone. It generally comes grey as standard but you can get it in black as well. And even white for that matter.
Most people only see the edge drips on a flat roof, new drips make it look clean and tidy.
Felt onto felt should always last longer because they expand and contract at the same rate in hot or cold weather.
goes without saying that a brand new roof will (should) last longer than a 10 year old one with some patches!
Could say they same presumably about replacing some slipped tiles on an old piched roof. New ones tend to last longer than old....
But even ignoring the difference in cost there are obvious reasons for fixing things rather than chucking them away and buying new!
Re using old tiles or slates if they have good life left makes sense.
Throwing good money after bad does not.
You are basically putting a sticking plaster on it .
That's fine to buy a bit of time , the final outcome is cast in stone