  Balmer-dominated shells in supernova remnants (SNRs) are produced by
collisionless shocks advancing into a partially neutral medium, and are most
frequently associated with Type Ia supernovae. We have analyzed Hubble Space
Telescope (HST) images and VLT/MUSE or AAT/WiFeS observations of five Type Ia
SNRs containing Balmer-dominated shells in the LMC: 0509-67.5, 0519-69.0,
N103B, DEM L71, and 0548-70.4. Contrary to expectations, we find bright
forbidden line emission from small dense knots embedded in four of these SNRs.
The electron densities in some knots are higher than 10$^4$ cm$^{-3}$. The size
and density of these knots are not characteristic for interstellar medium (ISM)
-- they most likely originate from a circumstellar medium (CSM) ejected by the
SN progenitor. Physical property variations of dense knots in the SNRs appear
to reflect an evolutionary effect. The recombination timescales for high
densities are short, and HST images of N103B taken 3.5 yr apart already show
brightness changes in some knots. VLT/MUSE observations detect [Fe XIV] line
emission from reverse shocks into SN ejecta as well as forward shocks into the
dense knots. Faint [O III] line emission is also detected from the Balmer shell
in 0519-69.0, N103B, and DEM L71. We exclude the postshock origin because the
[O III] line is narrow. For the preshock origin, we considered three
possibilities: photoionization precursor, cosmic ray precursor, and neutral
precursor. We conclude that the [O III] emission arises from oxygen that has
been photoionized by [He II] $\lambda$304 photons and is then collisionally
excited in a shock precursor heated mainly by cosmic rays.
