yookoso, you are right. 禁じる means 抑える or " to suppress", while 得ない means cannot or できない。禁じ得ない literally means 禁じることができない／抑えることができない or "unable to supress", but this grammar always used together with feelings such as 同情、驚き、失望、涙,....
Is this just the verb 禁じる + 得る [eru]? If so, should it have it's own entry when [eru] already exists?
I think the nuance is more towards "(I) can't control/supress". It is often used with 怒り,同情,驚き,失望,涙,笑い
is this the same as [tenaranai]
Tenaranaiis not the same as 禁じえない. 抑えられない can be the similar to 禁じえない. 禁じえない would be used when you talk about feelings. 同情を抑えられない。
[How would you change Ex.#5123 if you wanted to say:" He couldn't help being surprised by her." as opposed to the present meaning of "He couldn't help but surprise her."?]