Late romantic. With EH playing an expressive solo line of some lenght. Can a third chair handle this duty?
It depends on a few factors:
What is the level of player you are writing for? If professionals, the question of 'handling it' is irrelevant as any professional oboist is gonna be able to play both. If you are writing for students, you might wanna reconsider the use of Cor Anglais all together as they might not even own an English Horn.
Are you even having real people perform this (or is it just for an audio rendering). If you are actually preparing parts for real musicians to play, I would direct you away from N3 as the notation features are severely lacking in several base-line level aspects which make the parts that N3 outputs unusable in the average rehearsal setting. This applies especially when dealing with a professional orchestra as they are used to these base-lines and employ librarians whose job it is to make sure the parts are readable and to their standards. I have several horror stories I could tell you.
How much of the 3rd oboe part is actually Eng. Horn? Maybe just do away with the 3rd oboe part all together and make the entire part for English horn?
as for Strings; it only takes like 2-3 seconds to mute/un-mute their instruments. It is usually attached to their instrument and therefore is very easy to do without too much effort. Also, I highly recommend cracking a couple of musical tomes about orchestration (Kennan, Adler, even Berlioz) before jumping into full orchestral writing especially if you are gonna have live musicians play your music. N3 like all soft samples can lie.