Before we begin, I would like to point out that what we are discussing here is the authenticity rather than accuracy. Although the terms are usually used as synonyms, accuracy generally relates to the presentation of the facts as they are corroborated by the actual historical sources and thus is more a domain of documentaries rather than works of fiction. In the latter, it is authenticity that is usually invoked, meaning that although the characters, places and events might be completely fictional (such as character of the game's protagonist here), their milieu and actions could have existed as portrayed, because they map well to actual events that were documented in a given place and period. This might even extend to actual historical characters. For example, Hynce Ptáček (Hans Capon) is portrayed as a teenager in 1403, even though he was born around that time. But should he has been born 10-12 years earlier, his portrayal would fit the behaviour of a young man of his station. Thus, such portrayal is inaccurate, but still authentic. Having said that, let's move to the issues mentioned in the question.

I mentioned the situation of families and mobility in the Middle Ages in this recent entry, so I will limit myself to reiteration that being the only child and a son to boot, Jindřich (Henry in English version) was generally expected to live with his parents throughout his life to learn the craft and eventually inherit the homestead and forge to continue its operation in the service of the local lord, in this case, Bohdal of Drahenice (not mentioned in the game) and later Racek (Ratzig) Kobyla acting in the name of king after Skalice returned to the royal domain in 1403.

As for the rather casual contact between aforementioned noble and local blacksmith, this was something only to be expected. Feudal landowners residing in a given territory were usually granting land to their tenants on an individual basis and thus they were dealing with them directly, either in person or through assistants, depending on their availability. In addition, landowners were also often granted various policing, financial and judicial privileges, so they essentially were the people presiding over various legal issues their subjects might have had. This meant that such nobles were widely known by the local populace, or at least the heads of households renting the land and depending on their particular needs they might have been visiting them frequently, e.g. rural blacksmith would be responsible for providing and repairing equipment as well as making and fitting horseshoes for the castellan and his retinue, so quite familiar relation between sir Racek and Martin, Jindřich's father are nothing that could be considered unauthentic in early 15th century Europe.

Family of the main protagonist, much like all the inhabitants of Skalice are generally peasants that most likely held a relatively small plot of land, enough to provide sustenance for them (possibly in the ballpark of 20-30 acres) that is quite possibly relieved of all taxation in exchange for the provision of the specified services related to the craft of Martin, the head of the household. This would be in line with the locational privileges of 13th and 14th century Bohemia, Poland or Germany detailing the land grants and obligations of the tenants, where taxation of craftsmen was usually based on provision of specified goods and services to the landowner. So, in case of blacksmith this could mean e.g. a n agreed number of horseshoes and arrowhead as well as obligation to fit and replace the horseshoes whenever necessary.

The reluctance of Jindřich's parents to discuss marriage and future of their son might be explained by the piece information hidden in a spoiler below, it should be noted that the relation between Bianka and Jindřich is also not outstanding, as the personal and marital affairs among the peasants in early 15th century were highly diversified and often very informal. It is perfectly possible that Jindřich's parents are not that concerned given that he will inherit their entire homestead and no matter whom he will decide to marry, it should not influence their affairs. It is also plausible to think that they treat the relations between Jindřich and Bianka as the introduction to future marriage and do not give it that much thought - both families are well-respected and like each other, with Bianka's brother being Jindřich's friend.

this would rather not be a case between rather affluent peasants like the ones discussed here, the simple cohabitation of poorer people who did not have any wealth requiring pre-nuptial agreements were not unheard of in the Midle Ages, as were the marriages performed as a traditional lay ceremony, with the sacramental marriages before the priest becoming the largely dominant practice only in 14th Century in Bohemia and Poland (in the case of the latter, de facto lay unions were sometimes practiced in the eastern regions of the country even as late as mid-17th century). Please note that the marriage has been introduced to the list of sacraments only by Gregory IX in 1184, while the rules governing the administration of the sacrament that is largely associated with the Catholic marriage rites were drawn only at Council of Trent (1546-1563). But formalities notwithstanding, the subjects of sexual contacts between young people in folk songs and fables, as well as the recorded instances of extramarital pregnancies strongly suggest that the non-marital sexual contacts were far from absent in the discussed period.

Spoiler: Late in the game it is revealed that >!Jindřich is actually an illegitimate son of Racek Kobyla!< who, unable to marry low-born mother of his son due to his high position in the court of king Vaclav IV, made an agreement with Martin and was taking care of Jindřich's family since. This, although definitely not too common, was by all means possible and very well explains the relations between sir Racek and a common blacksmith.

So, to sum it up, the behaviours of various characters portrayed in 'Kingdom Come: Deliverance' are generally authentic, i.e. they fit into what could have been observed by a person living in this part of Bohemia in the early 14th century.