It's worth noting, first of all, that this question is somewhat anachronistic for Marx's own time, as income taxes were not generally levied, except under circumstances of war. The majority of taxation existed in the form of customs duties and property taxes. It's worth noting that these forms of taxation place the largest burden of the tax on the poor rather than the rich (this should be obvious for customs duties, as it is much the same as modern sales tax; that this applies to property taxes, one has to remember that the majority of such receipts would have been drawn from the ownership of land that was then rented out to be used; the tax burden, as with most rented property today, would have raised the rent). Nevertheless, it is worth noting that both Marx and Engels, at times, wrote in favour of progressive incomes taxes, as a means of shifting the burden of taxation from the poor to the rich.
At this point it seems possible to address your second question: given that the burden of taxation in Marx's time (and arguably our own) lies primarily on the poor, especially in the form of taxes which raise the cost of consumption, and given that the revenues of these taxes are used not for social programs (the welfare state does not yet exist at the time Marx is writing), but rather to fund wars and other actions of the state, taxation cannot be a means of returning value to labour; rather, taxation becomes yet another means of oppressing the working classes. Thus, in Capital Marx writes that:
The modern fiscal system, whose pivot is formed by taxes on the most
necessary means of subsistence (and therefore by increases in their
price), thus contains within itself the germ of automatic
progression. Over-taxation is not an accidental occurrence, but rather
a principle. In Holland, therefore, where this system was first
inaugurated, the great patriot, De Witt, extolled it in his Maxims as
the best system for making the wage-labourer submissive,
frugal, industrious...and overburdened with work. Here, however,
we are less concerned with the destructive influence it exercises on
the situation of the wage-labourer than with the forcible
expropriation, resulting from it, of peasants and artisans, in short,
of all the constituents of the lower middle class. There are no two
opinions about this, even among the bourgeois economists. Its
effectiveness as an expropriating agent is heightened still further
by the system of protection, which forms one of its integral parts.
(Marx, Capital, p. 921)
So why doesn't Marx portray profit as a wage paid to the owner of capital? In the "Preface to the First Edition," Marx sets his goal as analysing the "the social antagonisms that spring from the natural laws of capitalist production" (Marx, Capital, p. 91) In other words, Marx's aim is not to articulate how goods ought to be distributed---this being what much of modern economics is concerned with (given some definition of ought, i.e., to maximise overall societal wealth, etc.)---but rather, the laws by which capital functions, i.e., the assumptions which underlie a capitalist economy (Marx intends Capital to be a descriptive rather than normative study). From this angle, taxation is really of secondary importance to the more pressing question: how does there come to be something (value) to be taxed?
It is in explaining the genesis of value that Marx invokes the labour theory of value, which, as you pointed out, is not unique to Marx; in Marx's own view, he is simply taking over the account of the genesis of value given by the most able defenders of capitalism of his day. Thus, Marx's version of the labour theory of value is not terribly different from Ricardo's. But, then, if value is generated by labour, what is profit? Profit is evidently extracted from the surplus of value generated by labour, but if labour is the only thing which generates value, why should this surplus go to those who owns the means of production? (And this basic contradiction is, in part, why the labour theory of value fell into disuse; note that much of contemporary economics no longer concerns itself with how value is generated, but rather how it is distributed).