I don't a case for the [form[ - it's in regular use.  I think Sankoku's 文 is a bit overdone. The JEs have kana-only examples but the n-grams say otherwise.

@@ -18 +17,0 @@
-<misc>&form;</misc>

https://biz.trans-suite.jp/6674
「甚だ」の英語表現は「quite」「really」など
quite、really、greatly、highly、extremely、awfully、terribly
【副詞の使用例】
「甚だ疑問だ」という強く疑問を感じるという意味の表現
___ quite doubtful.
「甚だ遺憾（残念）」という残念な気持ちを強く伝える表現
___really regrettable.

As an apology, more likely written than not verbal form.  "very sorry"
「非常に申し訳ない」という気持ちを表す時に「甚だ申し訳ありません」と言うことができます。言葉で伝えるときよりも、書面で使う表現です。

https://eikaiwa.dmm.com/uknow/questions/100613/
"Its's extremely doubtful"＝「それは甚だ疑問である」

Another formal confirmation:
http://hinagata-shiritai.com/contents/detail/53/
非常にかしこまった、堅い印象を与える言葉としての意味も持っています。

Some randomly-selected [uk] comparisons　(kanji usually, not always wins)
甚だ疑問	49020
はなはだ疑問	27362

甚だ迷惑	4219
はなはだ迷惑	2957

甚だ残念	2580
はなはだ残念	3351

甚だ不安	2663
はなはだ不安	1593

甚だ怪しい	2063	  
はなはだ怪しい	1452

甚だ多い	997
はなはだ多い	693

Used heavily in formal apologies. 

Removed [uk],  adding [form]. Flagged as 文 in sankoku. Not generally [uk] in the ngrams; This looks like a common term where the kanji is commonly present.

"quite" and "highly" match [form] and are useful for common adjectives: "quite tall", "quite/highly doubtful", "quite/highly unlikely", etc. This is a common term (ichi1,news2,nf31) whose gloss depends highly on context, so I think a few more applicable suggestions won't hurt.

@@ -18 +18 @@
-<misc>&uk;</misc>
+<misc>&form;</misc>
@@ -20,0 +21 @@
+<gloss>extremely</gloss>
@@ -21,0 +23,2 @@
+<gloss>highly</gloss>
+<gloss>quite</gloss>

@@ -17,0 +18 @@
+<misc>&uk;</misc>