Possible Modal System

I am considering creating a modal system for a language that is rather
straightforwardly derived from an earlier, more isolating paradigm.  The
idea is that the modal suffixes are reduced forms of full verbs.  For
example, let there be a verbal root leka- meaning 'bind smth.' and a medio-
passive form leku- 'be bound.'  So, an obligative mood could be formed thus:

Haman senta kolesilkun.
Hama.n se.nta kole.si.lku.n
Something.sg.acc 3rd.sg.dat tell.irr.oblig.1sg
"I have to tell him something."

An earlier form would be: Hamam senta kolesi lekum.  It would literally
mean "I am bound to tell him something."

I'm assuming the irrealis form of the dependent verb should be used, since
semantically the action has not (yet) been performed.  Is this realistic?

Other possibilities:

Vane- "to be able" > -une- abilitative mood

Kalaten matsesiunesat.
Kala.t.en matse.si.une.sat
Fish.pl.acc catch.irr.abil.3pl
"They can catch (the) fish."

Puna- "will" > -mpa- volitive mood (via metathesis of *-pna-)

Sen cartasin junta kolesimpan.
Se.n carta.si.n ju.nta kole.si.mpa.n
3rd.sg.acc love.irr.1sg refl.dat tell.irr.vol.1sg
"I will tell her that I love her."

The question then arises, is it possible to form the conditional from the
irrealis form of the volitive?  Assuming that punasi- would mean "would,"
then the early form would be (of the same sentence above, except
conditional)

Sen cartasin junta kolesi punasin,

becoming something like

Sen cartasin junta kolesipunsin.

However, it seems redundant that there would be two instances of the
irrealis suffix -si in the final verb form.  Are there natural languages
that do this?

- Rob