I would like to understand the meaning and usage of those two sets of particles in here.
左の腋窩と右の脇腹に箆深く射された矢がなかったなら、 それはともすると羅馬の競技者が、 薄暮の庭樹に凭って疲れを休めている姿かとも見えた。
ともすると: I've read it works as an adverb "[subject] tends to/is prone to... [verb]". It's composed of とも, which basically means "even if" and could be replaced with やや as in ややすると, and すると "if [it is] done", just as すれば, in ともすれば, and したら, in ともしたら. I would like, however, a more literary translation of this adverb.
かとも: I think it has, not a conjunct, but a separate meaning, that would be [dubitation] + [conjunction-quotation] + [inclusion], and so, it can't be effectively translated. It marks a response to なかったなら in the first sentence.