Does pleading down to a lesser charge still leave me inadmissible to Canada?

It can, because what matters for immigration purposes is the offence you were actually convicted of, not the one you were originally charged with. If a plea deal results in a conviction for a lesser offence, an immigration officer assesses inadmissibility based on that final conviction and its Canadian equivalent — so a well-negotiated plea can genuinely reduce or eliminate an inadmissibility problem if it moves the conviction into a less serious category.

The complication is that officers aren't limited to the label of the final charge; they can look at the underlying facts on the record, including admissions made as part of the plea, to understand what actually happened. In some cases a technically lesser charge still corresponds to a serious Canadian equivalent once the real conduct is considered, so a plea deal doesn't automatically guarantee a better immigration outcome even though it may have resolved the criminal case favourably.

Anyone weighing a plea offer with future immigration plans in mind should get advice from an immigration lawyer, not just a criminal lawyer, before accepting it, since the two systems can score the same plea very differently.